SQL Constraints

Overview

SQL constraints are rules for data in a table. Constraints are used to prevent insertion of invalid data in a table, and ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted. Constraints can be specified in two ways: When a table is created (through the CREATE TABLE statement)

After a table is created

(through the ALTER TABLE
statement)

SQL Constraint Types

The following constraints are commonly used in SQL:

Not Null

Ensures that a column cannot have a NULL value

Unique

Ensures that all values in a column are unique

Primary Key

Uniquely identifies each row in a table (a combination of a NOT NULL and

Unique

)

Foreign Key

Establishes a link between data in two tables, and prevents action that will destroy the link between them

Check

Ensures that the values in a column satisfies a specific condition

Default

Sets a default value for a column if no value is specified

Create Index

Creates indexes on columns to retrieve data from the database faster

SQL ALTER TABLE Statement

SQL NOT NULL Constraint

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Learn/SQL/SQL Database

SQL•SQL Database

SQL Constraints

Overview

After a table is created

(through the ALTER TABLE
statement)

SQL Constraint Types

The following constraints are commonly used in SQL:

Not Null

Ensures that a column cannot have a NULL value

Unique

Ensures that all values in a column are unique

Primary Key

Uniquely identifies each row in a table (a combination of a NOT NULL and

Unique

)

Foreign Key

Establishes a link between data in two tables, and prevents action that will destroy the link between them

Check

Ensures that the values in a column satisfies a specific condition

Default

Sets a default value for a column if no value is specified

Create Index

Creates indexes on columns to retrieve data from the database faster

SQL ALTER TABLE Statement

SQL NOT NULL Constraint