SQL Basic
SQL HOME SQL Intro SQL Syntax SQL SELECT SQL SELECT DISTINCT SQL WHERE SQL AND & OR SQL ORDER BY SQL INSERT INTO SQL UPDATE SQL DELETESQL Advanced
SQL SELECT TOP SQL LIKE SQL Wildcards SQL IN SQL BETWEEN SQL Aliases SQL Joins SQL INNER JOIN SQL LEFT JOIN SQL RIGHT JOIN SQL FULL JOIN SQL UNION SQL SELECT INTO SQL INSERT INTO SELECT SQL CREATE DB SQL CREATE TABLE SQL Constraints SQL NOT NULL SQL UNIQUE SQL PRIMARY KEY SQL FOREIGN KEY SQL CHECK SQL DEFAULT SQL CREATE INDEX SQL DROP SQL ALTER SQL Auto Increment SQL Views SQL Dates SQL NULL Values SQL NULL Functions SQL General Data Types SQL DB Data TypesSQL Functions
SQL Functions SQL AVG() SQL COUNT() SQL FIRST() SQL LAST() SQL MAX() SQL MIN() SQL SUM() SQL GROUP BY SQL HAVING SQL UCASE() SQL LCASE() SQL MID() SQL LEN() SQL ROUND() SQL NOW() SQL FORMAT()SQL Quick Ref SQL Hosting SQL Summary
SQL Quiz
SQL QuizSQL Constraints
SQL Constraints
SQL constraints are used to specify rules for the data in a table.
If there is any violation between the constraint and the data action, the action is aborted by the constraint.
Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement).
SQL CREATE TABLE + CONSTRAINT Syntax
CREATE TABLE table_name
(
column_name1 data_type(size) constraint_name,
column_name2 data_type(size) constraint_name,
column_name3 data_type(size) constraint_name,
....
);
(
column_name1 data_type(size) constraint_name,
column_name2 data_type(size) constraint_name,
column_name3 data_type(size) constraint_name,
....
);
In SQL, we have the following constraints:
- NOT NULL - Indicates that a column cannot store NULL value
- UNIQUE - Ensures that each rows for a column must have a unique value
- PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly
- FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table
- CHECK - Ensures that the value in a column meets a specific condition
- DEFAULT - Specifies a default value when specified none for this column
The next chapters will describe each constraint in detail.
W3Schools' Online Certification
The perfect solution for professionals who need to balance work, family, and career building.
More than 10 000 certificates already issued!
The HTML Certificate documents your knowledge of HTML.
The HTML5 Certificate documents your knowledge of advanced HTML5.
The CSS Certificate documents your knowledge of advanced CSS.
The JavaScript Certificate documents your knowledge of JavaScript and HTML DOM.
The jQuery Certificate documents your knowledge of jQuery.
The XML Certificate documents your knowledge of XML, XML DOM and XSLT.
The ASP Certificate documents your knowledge of ASP, SQL, and ADO.
The PHP Certificate documents your knowledge of PHP and SQL (MySQL).
Your suggestion:
Close [X]
Thank You For Helping Us!
Your message has been sent to W3Schools.
Close [X]
