What are constraints in SQL?

Shripal Jain
Shripal Jain

Posted On: Dec 17, 2020


Constraints are the rules or limits that we can apply to the type of data in a table

The available constraints in SQL are:

NOT NULL: This constraint tells us that we cannot store a null value in a column.

UNIQUE: This constraint when specified with a column, tells that all the values in the column must be unique. 

PRIMARY KEY: A primary key is a field that can uniquely identify each row in a table. Typically, used to specify a field in a table as the primary key.

FOREIGN KEY: A Foreign key is a field that can uniquely identify each row in another table.

CHECK: This constraint helps to validate the values of a column to meet a particular condition. 

DEFAULT: This constraint specifies a default value for the column when no value is specified by the user.

For instance, 

ID int(6) NOT NULL,
NAME varchar(10) NOT NULL,
ADDRESS varchar(20)

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