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SQL Intro
An introduction to SQL.
SQL SELECT
How the SELECT statement is used to select data from a database.
SQL WHERE
How the WHERE clause is used to conditionally select data.
SQL INSERT INTO
How the INSERT INTO statement inserts new records (rows) into a database table.
SQL UPDATE
How to modify data in a database using the UPDATE statement.
SQL DELETE
How to delete records (rows) in a database, using the DELETE statement.
SQL Advanced
SQL ORDER BY
The ORDER BY keyword is used to sort the result.
SQL AND & OR
AND and OR join two or more conditions in a WHERE clause.
SQL IN
The IN operator may be used if you know the exact value(s) to search for.
SQL BETWEEN
The BETWEEN operator selects a range of data between two values.
SQL ALIAS
Aliases can be used for column names and table names.
SQL JOIN
How to use the JOIN operator to select data from two or more tables.
SQL UNION
The UNION operator is used to select related information from two tables.
SQL CREATE
How to create a new database and / or a new database table.
SQL DROP
How to drop (delete) a database and /or a database table.
SQL ALTER
How to user ALTER to add or delete table columns.
SQL Functions
SQL has a lot of built-in functions for counting and calculations.
SQL GROUP BY
How to use GROUP BY to group aggregated data.
SQL SELECT INTO
How to use SELECT INTO to save selected data in tables (create backup copies).
SQL CREATE VIEW
How to create a virtual table using CREATE VIEW.
SQL Server
About Database Management Systems (DBMS).
SQL Quick Reference
An SQL Quick Reference. Print it, fold it, and put it in your pocket.
SQL Summary
A summary of this tutorial.
SQL Quiz Test
Start SQL quiz!
Test your SQL skills at W3Schools!





SQL Functions
SQL AVG Function
The AVG function returns the average value of a column in a selection. NULL values are not included in the calculation.
Syntax
SELECT AVG(column) FROM table
Example
This example returns the average age of the persons in the "Persons" table:
SELECT AVG(Age) FROM Persons
Result
32.67
Example
This example returns the average age for persons that are older than 20 years:
SELECT AVG(Age) FROM Persons WHERE Age>20
Result
39.5

SQL COUNT Function
The COUNT(column) function returns the number of rows without a NULL value in the specified column.
Syntax
SELECT COUNT(column) FROM table
Example
With this "Persons" Table:
Name Age
Hansen, Ola 34
Svendson, Tove 45
Pettersen, Kari
This example finds the number of persons with a value in the "Age" field in the "Persons" table:
SELECT COUNT(Age) FROM Persons
Result:
2
The COUNT(column) function is handy for finding columns without a value. Note that the result is one less than the number of rows in the original table because one of the persons does not have an age value stored.
SQL COUNT(*) Function
The COUNT(*) function returns the number of selected rows in a selection.
Syntax
SELECT COUNT(*) FROM table
Example
With this "Persons" Table:
Name Age
Hansen, Ola 34
Svendson, Tove 45
Pettersen, Kari 19
This example returns the number of rows in the table:
SELECT COUNT(*) FROM Persons
Result:
3
Example
Return the number of persons that are older than 20 years:
SELECT COUNT(*) FROM Persons WHERE Age>20
Result:
2

SQL MAX Function
The MAX function returns the highest value in a column. NULL values are not included in the calculation.
Syntax
SELECT MAX(column) FROM table
Example
SELECT MAX(Age) FROM Persons
Result:
45
Note: The MIN and MAX functions can also be used on text columns, to find the highest or lowest value in alphabetical order.
SQL MIN Function
The MIN function returns the lowest value in a column. NULL values are not included in the calculation.
Syntax
SELECT MIN(column) FROM table
Example
SELECT MIN(Age) FROM Persons
Result:
19
Note: The MIN and MAX functions can also be used on text columns, to find the highest or lowest value in alphabetical order.
SQL SUM Function
The SUM function returns the total sum of a column in a given selection. NULL values are not included in the calculation.
Syntax
SELECT SUM(column) FROM table
Example
This example returns the sum of all ages in the "person" table:
SELECT SUM(Age) FROM Persons
Result:
98
Example
This example returns the sum of ages for persons that are more than 20 years old:
SELECT SUM(Age) FROM Persons WHERE Age>20
Result:
79

SQL AVG Function
The AVG function returns the average value of a column in a selection. NULL values are not included in the calculation.
Syntax
SELECT AVG(column) FROM table
Example
This example returns the average age of the persons in the "Persons" table:
SELECT AVG(Age) FROM Persons
Result
32.67
Example
This example returns the average age for persons that are older than 20 years:
SELECT AVG(Age) FROM Persons WHERE Age>20
Result
39.5

SQL COUNT Function
The COUNT(column) function returns the number of rows without a NULL value in the specified column.
Syntax
SELECT COUNT(column) FROM table
Example
With this "Persons" Table:
Name Age
Hansen, Ola 34
Svendson, Tove 45
Pettersen, Kari
This example finds the number of persons with a value in the "Age" field in the "Persons" table:
SELECT COUNT(Age) FROM Persons
Result:
2
The COUNT(column) function is handy for finding columns without a value. Note that the result is one less than the number of rows in the original table because one of the persons does not have an age value stored.
SQL COUNT DISTINCT Function
Note: The following example works with ORACLE and Microsoft SQL server but not with Microsoft Access.
The keyword DISTINCT and COUNT can be used together to count the number of distinct results.
Syntax
SELECT COUNT(DISTINCT column(s)) FROM table
Example
With this "Orders" Table:
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Example
SELECT COUNT(Company) FROM Orders
Result:
4
Example
SELECT COUNT(DISTINCT Company) FROM Orders
Result:
3

SQL FIRST Function
The FIRST function returns the value of the first record in the specified field.
Tip: Use the ORDER BY clause to order the records!
Syntax
SELECT FIRST(column) AS [expression]
FROM table
Example
SELECT FIRST(Age) AS lowest_age
FROM Persons
ORDER BY Age
Result:
19

SQL LAST Function
The LAST function returns the value of the last record in the specified field.
Tip: Use the ORDER BY clause to order the records!
Syntax
SELECT LAST(column) AS [expression]
FROM table
Example
SELECT LAST(Age) AS highest_age
FROM Persons
ORDER BY Age
Result:
45

SQL MAX Function
The MAX function returns the highest value in a column. NULL values are not included in the calculation.
Syntax
SELECT MAX(column) FROM table
Example
SELECT MAX(Age) FROM Persons
Result:
45
Note: The MIN and MAX functions can also be used on text columns, to find the highest or lowest value in alphabetical order.

SQL MIN Function
The MIN function returns the lowest value in a column. NULL values are not included in the calculation.
Syntax
SELECT MIN(column) FROM table
Example
SELECT MIN(Age) FROM Persons
Result:
19
Note: The MIN and MAX functions can also be used on text columns, to find the highest or lowest value in alphabetical order.

SQL SUM Function
The SUM function returns the total sum of a column in a given selection. NULL values are not included in the calculation.
Syntax
SELECT SUM(column) FROM table
Example
This example returns the sum of all ages in the "person" table:
SELECT SUM(Age) FROM Persons
Result:
98
Example
This example returns the sum of ages for persons that are more than 20 years old:
SELECT SUM(Age) FROM Persons WHERE Age>20
Result:
79


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