By 25/02/2016 5 Comments

What Are Constraints In SQL?

  • SQL constraints are different types of rules and restrictions which are applied in data in a database table.
  • If any violation action will be taken against the data, the constraints will prevent that action.
  • The SQL constraints can be specified at table creation time with CREATE TABLE statement or after table creation by using ALTER TABLE statement.


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

Following are SQL constraints:

Constraint Description
NOT NULL Indicates that a column cannot store NULL value. It must contains values.
UNIQUE Each row for a column must have a unique value. It has not duplicate values.
PRIMARY KEY It is used for uniquely identification for each record/row in a database table. It must be UNIQUE and NOT NULL.
FOREIGN KEY It is used for build a logically relationship between two tables. It enforces that the particular rows of both tables must be matched.
DEFAULT Set a default value for a column in database table. If you do not insert values in a table column the DEFAULT constraint sets default value for that column.
CHECK Ensures that the value in a column meets a specific condition.

Posted in: Sql

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