SQL is a standard language for storing, manipulating and retrieving data in databases.
Our SQL tutorial will teach you how to use SQL in: MySQL, SQL Server, MS Access, Oracle, Sybase, Informix, Postgres, and other database systems.
Start learning SQL now »
Examples in Each Chapter
With our online SQL editor, you can edit the SQL statements, and click on a button to view the result.
Click on the "Try it Yourself" button to see how it works.
SQL Exercises
Test Yourself With Exercises
Exercise:
Insert the missing statement to get all the columns from the Customers
table.
Start the Exercise
SQL Examples
Learn by examples! This tutorial supplements all explanations with clarifying examples.
See All SQL Examples
SQL Quiz Test
Test your SQL skills at W3Schools!
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My Learning
Track your progress with the free "My Learning" program here at W3Schools.
Log into your account, and start earning points!
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SQL References
At W3Schools you will find a complete reference for keywords and function:
SQL Keyword Reference
MYSQL Functions
SQLServer Functions
MS Access Functions
SQL Quick Reference
SQL Data Types
Data types and ranges for Microsoft Access, MySQL and SQL Server.
SQL Data Types
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w3schoolsCERTIFIED.2022MySQL is a widely used relational database management system [RDBMS].
MySQL is free and open-source.
MySQL is ideal for both small and large applications.
Start learning MySQL now »
Examples in Each Chapter
With our online MySQL editor, you can edit the SQL statements, and click on a button to view the result.
Click on the "Try it Yourself" button to see how it works.
MySQL Exercises
Test Yourself With Exercises
Exercise:
Insert the missing statement to get all the columns from the Customers
table.
Start the Exercise
MySQL Examples
Learn by examples! This tutorial supplements all explanations with clarifying examples.
See All SQL Examples
MySQL Quiz Test
Test your MySQL skills at W3Schools!
Start MySQL Quiz!
My Learning
Track your progress with the free "My Learning" program here at W3Schools.
Log into your account, and start earning points!
This is an optional feature, you can study W3Schools without using My Learning.
MySQL References
At W3Schools you will find a complete reference of MySQL data types and functions:
MySQL Data Types
MySQL Functions
AJAX can be used for interactive communication with a database.
AJAX Database Example
The following example will demonstrate how a web page can fetch information from a database with AJAX:
Example
Person info will be listed here...
Example Explained - The MySQL Database
The database table we use in the example above looks like this:
1 | Peter | Griffin | 41 | Quahog | Brewery |
2 | Lois | Griffin | 40 | Newport | Piano Teacher |
3 | Joseph | Swanson | 39 | Quahog | Police Officer |
4 | Glenn | Quagmire | 41 | Quahog | Pilot |
Example Explained
In the example above, when a user selects a person in the dropdown list above, a function called "showUser[]" is executed.
The function is triggered by the onchange event.
Here is the HTML code:
Example
function showUser[str] {
if [str == ""] {
document.getElementById["txtHint"].innerHTML = "";
return;
} else {
var xmlhttp = new XMLHttpRequest[];
xmlhttp.onreadystatechange = function[] {
if [this.readyState == 4 && this.status == 200] {
document.getElementById["txtHint"].innerHTML = this.responseText;
}
};
xmlhttp.open["GET","getuser.php?q="+str,true];
xmlhttp.send[];
}
}
Select a person:
Peter Griffin
Lois Griffin
Joseph Swanson
Glenn Quagmire
Run example »
Code explanation:
First, check if person is selected. If no person is selected [str == ""], clear the content of txtHint and exit the function. If a person is selected, do the following:
- Create an XMLHttpRequest object
- Create the function to be executed when the server response is ready
- Send the request off to a file on the server
- Notice that a parameter [q] is added to the URL [with the content of the dropdown list]
The PHP File
The page on the server called by the JavaScript above is a PHP file called "getuser.php".
The source code in "getuser.php" runs a query against a MySQL database, and returns the result in an HTML table:
table {
width: 100%;
border-collapse: collapse;
}
table, td, th {
border: 1px solid black;
padding: 5px;
}
th {text-align: left;}
Explanation: When the query is sent from the JavaScript to the PHP file, the following happens:
- PHP opens a connection to a MySQL server
- The correct person is found
- An HTML table is created, filled with data, and sent back to the "txtHint" placeholder