SQL Tutorial
SQL is a standard computer language for accessing and manipulating databases.
In this tutorial you will learn how to use SQL to access and manipulate data in Oracle, Sybase, SQL Server, DB2, Access, and other database systems.
Introduction to SQL
SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
SQL is a standard computer language for accessing and manipulating databases.
In this tutorial you will learn how to use SQL to access and manipulate data in Oracle, Sybase, SQL Server, DB2, Access, and other database systems.
Introduction to SQL
SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
Storgt 20 LastName
FirstName Address City
Hansen Ola Timoteivn 10
Sandnes Svendson Tove
Borgvn 23 Sandnes Pettersen
Kari
Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
SELECT - extracts data from a database table
UPDATE - updates data in a database table
DELETE - deletes data from a database table
INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
Introduction to SQL
SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
FirstName Address City
Hansen Ola Timoteivn 10
Sandnes Svendson Tove
Borgvn 23 Sandnes Pettersen
Kari
Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
SELECT - extracts data from a database table
UPDATE - updates data in a database table
DELETE - deletes data from a database table
INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
Introduction to SQL
SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
SELECT - extracts data from a database table
UPDATE - updates data in a database table
DELETE - deletes data from a database table
INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
SQL WHERE Clause
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
SELECT column FROM tableWHERE column operator value
With the WHERE clause, the following operators can be used:
Operator
Description
= Equal
<> Not equal
> Greater than
<>
>= Greater than or equal
<= Less than or equal BETWEEN Between an inclusive range LIKE Search for a pattern Note: In some versions of SQL the <> operator may be written as !=
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM PersonsWHERE City='Sandnes'
"Persons" table
<= Less than or equal BETWEEN Between an inclusive range LIKE Search for a pattern Note: In some versions of SQL the <> operator may be written as !=
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM PersonsWHERE City='Sandnes'
"Persons" table
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen KariStorgt 20
Stavanger
1960
Result
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen KariStorgt 20
Stavanger
1960
Result
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:SELECT * FROM Persons WHERE FirstName='Tove'This is wrong:SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct:SELECT * FROM Persons WHERE Year>1965This is wrong:SELECT * FROM Persons WHERE Year>'1965'
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM tableWHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM PersonsWHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM PersonsWHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM PersonsWHERE FirstName LIKE '%la%'
SQL INSERT INTO Statement
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_nameVALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)VALUES (value1, value2,....)
Insert a New Row
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
And this SQL statement:
INSERT INTO Persons VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Insert Data in Specified Columns
This "Persons" table:
LastName FirstName Address City
Svendson Stale Kaivn 18 Sandnes 1980
Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:SELECT * FROM Persons WHERE FirstName='Tove'This is wrong:SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct:SELECT * FROM Persons WHERE Year>1965This is wrong:SELECT * FROM Persons WHERE Year>'1965'
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM tableWHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM PersonsWHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM PersonsWHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM PersonsWHERE FirstName LIKE '%la%'
SQL INSERT INTO Statement
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_nameVALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)VALUES (value1, value2,....)
Insert a New Row
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
And this SQL statement:
INSERT INTO Persons VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Insert Data in Specified Columns
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
SQL UPDATE Statement
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_nameSET column_name = new_valueWHERE column_name = some_value
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Storgt 67
Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE PersonSET Address = 'Stien 12', City = 'Stavanger'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger
SQL DELETE Statement
The DELETE Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_nameWHERE column_name = some_value
Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger
Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Delete All Rows
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_nameorDELETE * FROM table_name
SQL ORDER BY
The ORDER BY keyword is used to sort the result.
Sort the Rows
The ORDER BY clause is used to sort the rows.
Orders:
Company
OrderNumber
Sega
3412
ABC Shop
5678
W3Schools
2312
W3Schools
6798
Example
To display the companies in alphabetical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company
Result:
Company
OrderNumber
ABC Shop
5678
Sega
3412
W3Schools
6798
W3Schools
2312
Example
To display the companies in alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company, OrderNumber
Result:
Company
OrderNumber
ABC Shop
5678
Sega
3412
W3Schools
2312
W3Schools
6798
Example
To display the companies in reverse alphabetical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company DESC
Result:
Company
OrderNumber
W3Schools
6798
W3Schools
2312
Sega
3412
ABC Shop
5678
Example
To display the companies in reverse alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company DESC, OrderNumber ASC
Result:
Company
OrderNumber
W3Schools
2312
W3Schools
6798
Sega
3412
ABC Shop
5678
SQL AND & OR
AND & OR
AND and OR join two or more conditions in a WHERE clause.
The AND operator displays a row if ALL conditions listed are true. The OR operator displays a row if ANY of the conditions listed are true.
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
Example
Use AND to display each person with the first name equal to "Tove", and the last name equal to "Svendson":
SELECT * FROM PersonsWHERE FirstName='Tove'AND LastName='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Example
Use OR to display each person with the first name equal to "Tove", or the last name equal to "Svendson":
SELECT * FROM PersonsWHERE firstname='Tove'OR lastname='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
Example
You can also combine AND and OR (use parentheses to form complex expressions):
SELECT * FROM Persons WHERE(FirstName='Tove' OR FirstName='Stephen')AND LastName='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
SQL IN
IN
The IN operator may be used if you know the exact value you want to return for at least one of the columns.
SELECT column_name FROM table_nameWHERE column_name IN (value1,value2,..)
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Nordmann
Anna
Neset 18
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
Svendson
Tove
Borgvn 23
Sandnes
Example 1
To display the persons with LastName equal to "Hansen" or "Pettersen", use the following SQL:
SELECT * FROM PersonsWHERE LastName IN ('Hansen','Pettersen')
Result:
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
SQL BETWEEN
BETWEEN ... AND
The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.
SELECT column_name FROM table_nameWHERE column_nameBETWEEN value1 AND value2
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Nordmann
Anna
Neset 18
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
Svendson
Tove
Borgvn 23
Sandnes
SQL Alias
With SQL, aliases can be used for column names and table names.
Column Name Alias
The syntax is:
SELECT column AS column_alias FROM table
Table Name Alias
The syntax is:
SELECT column FROM table AS table_alias
Example: Using a Column Alias
This table (Persons):
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
And this SQL:
SELECT LastName AS Family, FirstName AS NameFROM Persons
Returns this result:
Family
Name
Hansen
Ola
Svendson
Tove
Pettersen
Kari
Example: Using a Table Alias
This table (Persons):
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
And this SQL:
SELECT LastName, FirstNameFROM Persons AS Employees
Returns this result:
Table Employees:
LastName
FirstName
Hansen
Ola
Svendson
Tove
Pettersen
Kari
SQL JOIN
Joins and Keys
Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:
The "Employee_ID" column is the primary key of the "Employees" table
The "Prod_ID" column is the primary key of the "Orders" table
The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names
Employees:
Employee_ID
Name
01
Hansen, Ola
02
Svendson, Tove
03
Svendson, Stephen
04
Pettersen, Kari
Orders:
Prod_ID
Product
Employee_ID
234
Printer
01
657
Table
03
865
Chair
03
Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.ProductFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_ID
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example
Who ordered a printer?
SELECT Employees.NameFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_IDAND Orders.Product='Printer'
Result
Name
Hansen, Ola
Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax
SELECT field1, field2, field3FROM first_tableINNER JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.ProductFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example LEFT JOIN
Syntax
SELECT field1, field2, field3FROM first_tableLEFT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
List all employees, and their orders - if any.
SELECT Employees.Name, Orders.ProductFROM EmployeesLEFT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Tove
Svendson, Stephen
Table
Svendson, Stephen
Chair
Pettersen, Kari
Example RIGHT JOIN
Syntax
SELECT field1, field2, field3FROM first_tableRIGHT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
List all orders, and who has ordered - if any.
SELECT Employees.Name, Orders.ProductFROM EmployeesRIGHT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example
Who ordered a printer?
SELECT Employees.NameFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_IDWHERE Orders.Product = 'Printer'
Result
Name
Hansen, Ola
SQL UNION and UNION ALL
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
Note: With UNION, only distinct values are selected.
SQL Statement 1UNIONSQL Statement 2
Employees_Norway:
E_ID
E_Name
01
Hansen, Ola
02
Svendson, Tove
03
Svendson, Stephen
04
Pettersen, Kari
Employees_USA:
E_ID
E_Name
01
Turner, Sally
02
Kent, Clark
03
Svendson, Stephen
04
Scott, Stephen
Using the UNION Command
Example
List all different employee names in Norway and USA:
SELECT E_Name FROM Employees_NorwayUNIONSELECT E_Name FROM Employees_USA
Result
E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Scott, Stephen
Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
SQL Statement 1UNION ALLSQL Statement 2
Using the UNION ALL Command
Example
List all employees in Norway and USA:
SELECT E_Name FROM Employees_NorwayUNION ALLSELECT E_Name FROM Employees_USA
Result
E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Svendson, Stephen
Scott, Stephen
SQL Create Database, Table, and Index
Create a Database
To create a database:
CREATE DATABASE database_name
Create a Table
To create a table in a database:
CREATE TABLE table_name(column_name1 data_type,column_name2 data_type,.......)
Example
This example demonstrates how you can create a table named "Person", with four columns. The column names will be "LastName", "FirstName", "Address", and "Age":
CREATE TABLE Person (LastName varchar,FirstName varchar,Address varchar,Age int)
This example demonstrates how you can specify a maximum length for some columns:
CREATE TABLE Person (LastName varchar(30),FirstName varchar,Address varchar,Age int(3) )
The data type specifies what type of data the column can hold. The table below contains the most common data types in SQL:
Data Type
Description
integer(size)int(size)smallint(size)tinyint(size)
Hold integers only. The maximum number of digits are specified in parenthesis.
decimal(size,d)numeric(size,d)
Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d".
char(size)
Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis.
varchar(size)
Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis.
date(yyyymmdd)
Holds a date
Create Index
Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries.
Note: Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update. So, it is a good idea to create indexes only on columns that are often used for a search.
A Unique Index
Creates a unique index on a table. A unique index means that two rows cannot have the same index value.
CREATE UNIQUE INDEX index_nameON table_name (column_name)
The "column_name" specifies the column you want indexed.
A Simple Index
Creates a simple index on a table. When the UNIQUE keyword is omitted, duplicate values are allowed.
CREATE INDEX index_nameON table_name (column_name)
The "column_name" specifies the column you want indexed.
Example
This example creates a simple index, named "PersonIndex", on the LastName field of the Person table:
CREATE INDEX PersonIndexON Person (LastName)
If you want to index the values in a column in descending order, you can add the reserved word DESC after the column name:
CREATE INDEX PersonIndexON Person (LastName DESC)
If you want to index more than one column you can list the column names within the parentheses, separated by commas:
CREATE INDEX PersonIndexON Person (LastName, FirstName)
SQL Drop Index, Table and Database
Drop Index
You can delete an existing index in a table with the DROP INDEX statement.
Syntax for Microsoft SQLJet (and Microsoft Access):
DROP INDEX index_name ON table_name
Syntax for MS SQL Server:
DROP INDEX table_name.index_name
Syntax for IBM DB2 and Oracle:
DROP INDEX index_name
Syntax for MySQL:
ALTER TABLE table_name DROP INDEX index_name
Delete a Table or Database
To delete a table (the table structure, attributes, and indexes will also be deleted):
DROP TABLE table_name
To delete a database:
DROP DATABASE database_name
Truncate a Table
What if we only want to get rid of the data inside a table, and not the table itself? Use the TRUNCATE TABLE command (deletes only the data inside the table):
TRUNCATE TABLE table_name
SQL ALTER TABLE
ALTER TABLE
The ALTER TABLE statement is used to add or drop columns in an existing table.
ALTER TABLE table_name ADD column_name datatypeALTER TABLE table_name DROP COLUMN column_name
Note: Some database systems don't allow the dropping of a column in a database table (DROP COLUMN column_name).
Person:
LastName
FirstName
Address
Pettersen
Kari
Storgt 20
Example
To add a column named "City" in the "Person" table:
ALTER TABLE Person ADD City varchar(30)
Result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Example
To drop the "Address" column in the "Person" table:
ALTER TABLE Person DROP COLUMN Address
Result:
LastName
FirstName
City
Pettersen
Kari
SQL Functions
SQL has a lot of built-in functions for counting and calculations.
Function Syntax
The syntax for built-in SQL functions is:
SELECT function(column) FROM table
Types of Functions
There are several basic types and categories of functions in SQL. The basic types of functions are:
Aggregate Functions
Scalar functions
Aggregate functions
Aggregate functions operate against a collection of values, but return a single value.
Note: If used among many other expressions in the item list of a SELECT statement, the SELECT must have a GROUP BY clause!!
"Persons" table (used in most examples)
Name
Age
Hansen, Ola
34
Svendson, Tove
45
Pettersen, Kari
19
Aggregate functions in MS Access
Function
Description
AVG(column)
Returns the average value of a column
COUNT(column)
Returns the number of rows (without a NULL value) of a column
COUNT(*)
Returns the number of selected rows
FIRST(column)
Returns the value of the first record in a specified field
LAST(column)
Returns the value of the last record in a specified field
MAX(column)
Returns the highest value of a column
MIN(column)
Returns the lowest value of a column
STDEV(column)
STDEVP(column)
SUM(column)
Returns the total sum of a column
VAR(column)
VARP(column)
Aggregate functions in SQL Server
Function
Description
AVG(column)
Returns the average value of a column
BINARY_CHECKSUM
CHECKSUM
CHECKSUM_AGG
COUNT(column)
Returns the number of rows (without a NULL value) of a column
COUNT(*)
Returns the number of selected rows
COUNT(DISTINCT column)
Returns the number of distinct results
FIRST(column)
Returns the value of the first record in a specified field (not supported in SQLServer2K)
LAST(column)
Returns the value of the last record in a specified field (not supported in SQLServer2K)
MAX(column)
Returns the highest value of a column
MIN(column)
Returns the lowest value of a column
STDEV(column)
STDEVP(column)
SUM(column)
Returns the total sum of a column
VAR(column)
VARP(column)
Scalar functions
Scalar functions operate against a single value, and return a single value based on the input value.
Useful Scalar Functions in MS Access
Function
Description
UCASE(c)
Converts a field to upper case
LCASE(c)
Converts a field to lower case
MID(c,start[,end])
Extract characters from a text field
LEN(c)
Returns the length of a text field
INSTR(c,char)
Returns the numeric position of a named character within a text field
LEFT(c,number_of_char)
Return the left part of a text field requested
RIGHT(c,number_of_char)
Return the right part of a text field requested
ROUND(c,decimals)
Rounds a numeric field to the number of decimals specified
MOD(x,y)
Returns the remainder of a division operation
NOW()
Returns the current system date
FORMAT(c,format)
Changes the way a field is displayed
DATEDIFF(d,date1,date2)
Used to perform date calculations
SQL GROUP BY and HAVING
Aggregate functions (like SUM) often need an added GROUP BY functionality.
GROUP BY...
GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.
The syntax for the GROUP BY function is:
SELECT column,SUM(column) FROM table GROUP BY column
GROUP BY Example
This "Sales" Table:
Company
Amount
W3Schools
5500
IBM
4500
W3Schools
7100
And This SQL:
SELECT Company, SUM(Amount) FROM Sales
Returns this result:
Company
SUM(Amount)
W3Schools
17100
IBM
17100
W3Schools
17100
The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:
SELECT Company,SUM(Amount) FROM SalesGROUP BY Company
Returns this result:
Company
SUM(Amount)
W3Schools
12600
IBM
4500
HAVING...
HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.
The syntax for the HAVING function is:
SELECT column,SUM(column) FROM tableGROUP BY columnHAVING SUM(column) condition value
This "Sales" Table:
Company
Amount
W3Schools
5500
IBM
4500
W3Schools
7100
This SQL:
SELECT Company,SUM(Amount) FROM SalesGROUP BY CompanyHAVING SUM(Amount)>10000
Returns this result
Company
SUM(Amount)
W3Schools
12600
SQL SELECT INTO Statement
The SELECT INTO Statement
The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.
Syntax
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
Make a Backup Copy
The following example makes a backup copy of the "Persons" table:
SELECT * INTO Persons_backupFROM Persons
The IN clause can be used to copy tables into another database:
SELECT Persons.* INTO Persons IN 'Backup.mdb'FROM Persons
If you only want to copy a few fields, you can do so by listing them after the SELECT statement:
SELECT LastName,FirstName INTO Persons_backupFROM Persons
You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:
SELECT LastName,Firstname INTO Persons_backupFROM PersonsWHERE City='Sandnes'
Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:
SELECT Employees.Name,Orders.ProductINTO Empl_Ord_backupFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
SQL CREATE VIEW Statement
A view is a virtual table based on the result-set of a SELECT statement.
What is a View?
In SQL, a VIEW is a virtual table based on the result-set of a SELECT statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from a single table.
Note: The database design and structure will NOT be affected by the functions, where, or join statements in a view.
Syntax
CREATE VIEW view_name ASSELECT column_name(s)FROM table_nameWHERE condition
Note: The database does not store the view data! The database engine recreates the data, using the view's SELECT statement, every time a user queries a view.
Using Views
A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc., to a view, it allows you to present exactly the data you want to the user.
The sample database Northwind has some views installed by default. The view "Current Product List" lists all active products (products that are not discontinued) from the Products table. The view is created with the following SQL:
CREATE VIEW [Current Product List] ASSELECT ProductID,ProductNameFROM ProductsWHERE Discontinued=No
We can query the view above as follows:
SELECT * FROM [Current Product List]
Another view from the Northwind sample database selects every product in the Products table that has a unit price that is higher than the average unit price:
CREATE VIEW [Products Above Average Price] ASSELECT ProductName,UnitPriceFROM ProductsWHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:
SELECT * FROM [Products Above Average Price]
Another example view from the Northwind database calculates the total sale for each category in 1997. Note that this view select its data from another view called "Product Sales for 1997":
CREATE VIEW [Category Sales For 1997] ASSELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySalesFROM [Product Sales for 1997]GROUP BY CategoryName
We can query the view above as follows:
SELECT * FROM [Category Sales For 1997]
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
SELECT * FROM [Category Sales For 1997]WHERE CategoryName='Beverages'
SQL Servers - RDBMS
Modern SQL Servers are built on RDBMS.
DBMS - Database Management System
A Database Management System (DBMS) is a computer program that can access data in a database.
The DBMS program enables you to extract, modify, or store information in a database.
Different DBMS programs provides different functions for querying data, reporting data, and modifying data.
RDBMS - Relational Database Management System
A Relational Database Management System (RDBMS) is a Database Management System (DBMS) where the database is organized and accessed according to the relationships between data.
RDBMS was invented by IBM in the early 1970's.
RDBMS is the basis for SQL, and for all modern database systems like Oracle, SQL Server, IBM DB2, Sybase, MySQL, and Microsoft Access.
Hetland Camilla Hagabakka 24 Sandnes
SQL UPDATE Statement
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_nameSET column_name = new_valueWHERE column_name = some_value
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Storgt 67
Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE PersonSET Address = 'Stien 12', City = 'Stavanger'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger
SQL DELETE Statement
The DELETE Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_nameWHERE column_name = some_value
Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger
Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Delete All Rows
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_nameorDELETE * FROM table_name
SQL ORDER BY
The ORDER BY keyword is used to sort the result.
Sort the Rows
The ORDER BY clause is used to sort the rows.
Orders:
Company
OrderNumber
Sega
3412
ABC Shop
5678
W3Schools
2312
W3Schools
6798
Example
To display the companies in alphabetical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company
Result:
Company
OrderNumber
ABC Shop
5678
Sega
3412
W3Schools
6798
W3Schools
2312
Example
To display the companies in alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company, OrderNumber
Result:
Company
OrderNumber
ABC Shop
5678
Sega
3412
W3Schools
2312
W3Schools
6798
Example
To display the companies in reverse alphabetical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company DESC
Result:
Company
OrderNumber
W3Schools
6798
W3Schools
2312
Sega
3412
ABC Shop
5678
Example
To display the companies in reverse alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM OrdersORDER BY Company DESC, OrderNumber ASC
Result:
Company
OrderNumber
W3Schools
2312
W3Schools
6798
Sega
3412
ABC Shop
5678
SQL AND & OR
AND & OR
AND and OR join two or more conditions in a WHERE clause.
The AND operator displays a row if ALL conditions listed are true. The OR operator displays a row if ANY of the conditions listed are true.
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
Example
Use AND to display each person with the first name equal to "Tove", and the last name equal to "Svendson":
SELECT * FROM PersonsWHERE FirstName='Tove'AND LastName='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Example
Use OR to display each person with the first name equal to "Tove", or the last name equal to "Svendson":
SELECT * FROM PersonsWHERE firstname='Tove'OR lastname='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
Example
You can also combine AND and OR (use parentheses to form complex expressions):
SELECT * FROM Persons WHERE(FirstName='Tove' OR FirstName='Stephen')AND LastName='Svendson'
Result:
LastName
FirstName
Address
City
Svendson
Tove
Borgvn 23
Sandnes
Svendson
Stephen
Kaivn 18
Sandnes
SQL IN
IN
The IN operator may be used if you know the exact value you want to return for at least one of the columns.
SELECT column_name FROM table_nameWHERE column_name IN (value1,value2,..)
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Nordmann
Anna
Neset 18
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
Svendson
Tove
Borgvn 23
Sandnes
Example 1
To display the persons with LastName equal to "Hansen" or "Pettersen", use the following SQL:
SELECT * FROM PersonsWHERE LastName IN ('Hansen','Pettersen')
Result:
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
SQL BETWEEN
BETWEEN ... AND
The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.
SELECT column_name FROM table_nameWHERE column_nameBETWEEN value1 AND value2
Original Table (used in the examples)
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Nordmann
Anna
Neset 18
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
Svendson
Tove
Borgvn 23
Sandnes
SQL Alias
With SQL, aliases can be used for column names and table names.
Column Name Alias
The syntax is:
SELECT column AS column_alias FROM table
Table Name Alias
The syntax is:
SELECT column FROM table AS table_alias
Example: Using a Column Alias
This table (Persons):
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
And this SQL:
SELECT LastName AS Family, FirstName AS NameFROM Persons
Returns this result:
Family
Name
Hansen
Ola
Svendson
Tove
Pettersen
Kari
Example: Using a Table Alias
This table (Persons):
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
And this SQL:
SELECT LastName, FirstNameFROM Persons AS Employees
Returns this result:
Table Employees:
LastName
FirstName
Hansen
Ola
Svendson
Tove
Pettersen
Kari
SQL JOIN
Joins and Keys
Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:
The "Employee_ID" column is the primary key of the "Employees" table
The "Prod_ID" column is the primary key of the "Orders" table
The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names
Employees:
Employee_ID
Name
01
Hansen, Ola
02
Svendson, Tove
03
Svendson, Stephen
04
Pettersen, Kari
Orders:
Prod_ID
Product
Employee_ID
234
Printer
01
657
Table
03
865
Chair
03
Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.ProductFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_ID
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example
Who ordered a printer?
SELECT Employees.NameFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_IDAND Orders.Product='Printer'
Result
Name
Hansen, Ola
Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax
SELECT field1, field2, field3FROM first_tableINNER JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.ProductFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example LEFT JOIN
Syntax
SELECT field1, field2, field3FROM first_tableLEFT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
List all employees, and their orders - if any.
SELECT Employees.Name, Orders.ProductFROM EmployeesLEFT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Tove
Svendson, Stephen
Table
Svendson, Stephen
Chair
Pettersen, Kari
Example RIGHT JOIN
Syntax
SELECT field1, field2, field3FROM first_tableRIGHT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield
List all orders, and who has ordered - if any.
SELECT Employees.Name, Orders.ProductFROM EmployeesRIGHT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result
Name
Product
Hansen, Ola
Printer
Svendson, Stephen
Table
Svendson, Stephen
Chair
Example
Who ordered a printer?
SELECT Employees.NameFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_IDWHERE Orders.Product = 'Printer'
Result
Name
Hansen, Ola
SQL UNION and UNION ALL
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
Note: With UNION, only distinct values are selected.
SQL Statement 1UNIONSQL Statement 2
Employees_Norway:
E_ID
E_Name
01
Hansen, Ola
02
Svendson, Tove
03
Svendson, Stephen
04
Pettersen, Kari
Employees_USA:
E_ID
E_Name
01
Turner, Sally
02
Kent, Clark
03
Svendson, Stephen
04
Scott, Stephen
Using the UNION Command
Example
List all different employee names in Norway and USA:
SELECT E_Name FROM Employees_NorwayUNIONSELECT E_Name FROM Employees_USA
Result
E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Scott, Stephen
Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
SQL Statement 1UNION ALLSQL Statement 2
Using the UNION ALL Command
Example
List all employees in Norway and USA:
SELECT E_Name FROM Employees_NorwayUNION ALLSELECT E_Name FROM Employees_USA
Result
E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Svendson, Stephen
Scott, Stephen
SQL Create Database, Table, and Index
Create a Database
To create a database:
CREATE DATABASE database_name
Create a Table
To create a table in a database:
CREATE TABLE table_name(column_name1 data_type,column_name2 data_type,.......)
Example
This example demonstrates how you can create a table named "Person", with four columns. The column names will be "LastName", "FirstName", "Address", and "Age":
CREATE TABLE Person (LastName varchar,FirstName varchar,Address varchar,Age int)
This example demonstrates how you can specify a maximum length for some columns:
CREATE TABLE Person (LastName varchar(30),FirstName varchar,Address varchar,Age int(3) )
The data type specifies what type of data the column can hold. The table below contains the most common data types in SQL:
Data Type
Description
integer(size)int(size)smallint(size)tinyint(size)
Hold integers only. The maximum number of digits are specified in parenthesis.
decimal(size,d)numeric(size,d)
Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d".
char(size)
Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis.
varchar(size)
Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis.
date(yyyymmdd)
Holds a date
Create Index
Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries.
Note: Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update. So, it is a good idea to create indexes only on columns that are often used for a search.
A Unique Index
Creates a unique index on a table. A unique index means that two rows cannot have the same index value.
CREATE UNIQUE INDEX index_nameON table_name (column_name)
The "column_name" specifies the column you want indexed.
A Simple Index
Creates a simple index on a table. When the UNIQUE keyword is omitted, duplicate values are allowed.
CREATE INDEX index_nameON table_name (column_name)
The "column_name" specifies the column you want indexed.
Example
This example creates a simple index, named "PersonIndex", on the LastName field of the Person table:
CREATE INDEX PersonIndexON Person (LastName)
If you want to index the values in a column in descending order, you can add the reserved word DESC after the column name:
CREATE INDEX PersonIndexON Person (LastName DESC)
If you want to index more than one column you can list the column names within the parentheses, separated by commas:
CREATE INDEX PersonIndexON Person (LastName, FirstName)
SQL Drop Index, Table and Database
Drop Index
You can delete an existing index in a table with the DROP INDEX statement.
Syntax for Microsoft SQLJet (and Microsoft Access):
DROP INDEX index_name ON table_name
Syntax for MS SQL Server:
DROP INDEX table_name.index_name
Syntax for IBM DB2 and Oracle:
DROP INDEX index_name
Syntax for MySQL:
ALTER TABLE table_name DROP INDEX index_name
Delete a Table or Database
To delete a table (the table structure, attributes, and indexes will also be deleted):
DROP TABLE table_name
To delete a database:
DROP DATABASE database_name
Truncate a Table
What if we only want to get rid of the data inside a table, and not the table itself? Use the TRUNCATE TABLE command (deletes only the data inside the table):
TRUNCATE TABLE table_name
SQL ALTER TABLE
ALTER TABLE
The ALTER TABLE statement is used to add or drop columns in an existing table.
ALTER TABLE table_name ADD column_name datatypeALTER TABLE table_name DROP COLUMN column_name
Note: Some database systems don't allow the dropping of a column in a database table (DROP COLUMN column_name).
Person:
LastName
FirstName
Address
Pettersen
Kari
Storgt 20
Example
To add a column named "City" in the "Person" table:
ALTER TABLE Person ADD City varchar(30)
Result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Example
To drop the "Address" column in the "Person" table:
ALTER TABLE Person DROP COLUMN Address
Result:
LastName
FirstName
City
Pettersen
Kari
SQL Functions
SQL has a lot of built-in functions for counting and calculations.
Function Syntax
The syntax for built-in SQL functions is:
SELECT function(column) FROM table
Types of Functions
There are several basic types and categories of functions in SQL. The basic types of functions are:
Aggregate Functions
Scalar functions
Aggregate functions
Aggregate functions operate against a collection of values, but return a single value.
Note: If used among many other expressions in the item list of a SELECT statement, the SELECT must have a GROUP BY clause!!
"Persons" table (used in most examples)
Name
Age
Hansen, Ola
34
Svendson, Tove
45
Pettersen, Kari
19
Aggregate functions in MS Access
Function
Description
AVG(column)
Returns the average value of a column
COUNT(column)
Returns the number of rows (without a NULL value) of a column
COUNT(*)
Returns the number of selected rows
FIRST(column)
Returns the value of the first record in a specified field
LAST(column)
Returns the value of the last record in a specified field
MAX(column)
Returns the highest value of a column
MIN(column)
Returns the lowest value of a column
STDEV(column)
STDEVP(column)
SUM(column)
Returns the total sum of a column
VAR(column)
VARP(column)
Aggregate functions in SQL Server
Function
Description
AVG(column)
Returns the average value of a column
BINARY_CHECKSUM
CHECKSUM
CHECKSUM_AGG
COUNT(column)
Returns the number of rows (without a NULL value) of a column
COUNT(*)
Returns the number of selected rows
COUNT(DISTINCT column)
Returns the number of distinct results
FIRST(column)
Returns the value of the first record in a specified field (not supported in SQLServer2K)
LAST(column)
Returns the value of the last record in a specified field (not supported in SQLServer2K)
MAX(column)
Returns the highest value of a column
MIN(column)
Returns the lowest value of a column
STDEV(column)
STDEVP(column)
SUM(column)
Returns the total sum of a column
VAR(column)
VARP(column)
Scalar functions
Scalar functions operate against a single value, and return a single value based on the input value.
Useful Scalar Functions in MS Access
Function
Description
UCASE(c)
Converts a field to upper case
LCASE(c)
Converts a field to lower case
MID(c,start[,end])
Extract characters from a text field
LEN(c)
Returns the length of a text field
INSTR(c,char)
Returns the numeric position of a named character within a text field
LEFT(c,number_of_char)
Return the left part of a text field requested
RIGHT(c,number_of_char)
Return the right part of a text field requested
ROUND(c,decimals)
Rounds a numeric field to the number of decimals specified
MOD(x,y)
Returns the remainder of a division operation
NOW()
Returns the current system date
FORMAT(c,format)
Changes the way a field is displayed
DATEDIFF(d,date1,date2)
Used to perform date calculations
SQL GROUP BY and HAVING
Aggregate functions (like SUM) often need an added GROUP BY functionality.
GROUP BY...
GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.
The syntax for the GROUP BY function is:
SELECT column,SUM(column) FROM table GROUP BY column
GROUP BY Example
This "Sales" Table:
Company
Amount
W3Schools
5500
IBM
4500
W3Schools
7100
And This SQL:
SELECT Company, SUM(Amount) FROM Sales
Returns this result:
Company
SUM(Amount)
W3Schools
17100
IBM
17100
W3Schools
17100
The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:
SELECT Company,SUM(Amount) FROM SalesGROUP BY Company
Returns this result:
Company
SUM(Amount)
W3Schools
12600
IBM
4500
HAVING...
HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.
The syntax for the HAVING function is:
SELECT column,SUM(column) FROM tableGROUP BY columnHAVING SUM(column) condition value
This "Sales" Table:
Company
Amount
W3Schools
5500
IBM
4500
W3Schools
7100
This SQL:
SELECT Company,SUM(Amount) FROM SalesGROUP BY CompanyHAVING SUM(Amount)>10000
Returns this result
Company
SUM(Amount)
W3Schools
12600
SQL SELECT INTO Statement
The SELECT INTO Statement
The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.
Syntax
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
Make a Backup Copy
The following example makes a backup copy of the "Persons" table:
SELECT * INTO Persons_backupFROM Persons
The IN clause can be used to copy tables into another database:
SELECT Persons.* INTO Persons IN 'Backup.mdb'FROM Persons
If you only want to copy a few fields, you can do so by listing them after the SELECT statement:
SELECT LastName,FirstName INTO Persons_backupFROM Persons
You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:
SELECT LastName,Firstname INTO Persons_backupFROM PersonsWHERE City='Sandnes'
Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:
SELECT Employees.Name,Orders.ProductINTO Empl_Ord_backupFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID
SQL CREATE VIEW Statement
A view is a virtual table based on the result-set of a SELECT statement.
What is a View?
In SQL, a VIEW is a virtual table based on the result-set of a SELECT statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from a single table.
Note: The database design and structure will NOT be affected by the functions, where, or join statements in a view.
Syntax
CREATE VIEW view_name ASSELECT column_name(s)FROM table_nameWHERE condition
Note: The database does not store the view data! The database engine recreates the data, using the view's SELECT statement, every time a user queries a view.
Using Views
A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc., to a view, it allows you to present exactly the data you want to the user.
The sample database Northwind has some views installed by default. The view "Current Product List" lists all active products (products that are not discontinued) from the Products table. The view is created with the following SQL:
CREATE VIEW [Current Product List] ASSELECT ProductID,ProductNameFROM ProductsWHERE Discontinued=No
We can query the view above as follows:
SELECT * FROM [Current Product List]
Another view from the Northwind sample database selects every product in the Products table that has a unit price that is higher than the average unit price:
CREATE VIEW [Products Above Average Price] ASSELECT ProductName,UnitPriceFROM ProductsWHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:
SELECT * FROM [Products Above Average Price]
Another example view from the Northwind database calculates the total sale for each category in 1997. Note that this view select its data from another view called "Product Sales for 1997":
CREATE VIEW [Category Sales For 1997] ASSELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySalesFROM [Product Sales for 1997]GROUP BY CategoryName
We can query the view above as follows:
SELECT * FROM [Category Sales For 1997]
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
SELECT * FROM [Category Sales For 1997]WHERE CategoryName='Beverages'
SQL Servers - RDBMS
Modern SQL Servers are built on RDBMS.
DBMS - Database Management System
A Database Management System (DBMS) is a computer program that can access data in a database.
The DBMS program enables you to extract, modify, or store information in a database.
Different DBMS programs provides different functions for querying data, reporting data, and modifying data.
RDBMS - Relational Database Management System
A Relational Database Management System (RDBMS) is a Database Management System (DBMS) where the database is organized and accessed according to the relationships between data.
RDBMS was invented by IBM in the early 1970's.
RDBMS is the basis for SQL, and for all modern database systems like Oracle, SQL Server, IBM DB2, Sybase, MySQL, and Microsoft Access.
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tips and tricks are more interested than history...
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