What is Primary Key in SQL ?

Primary Key is a constraint in SQL. So, before understanding what exactly is a primary key, let’s understand what exactly is a constraint in SQL. Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. Constraints can either be column level or table level.

Let’s look at the different types of constraints which are present in SQL:

Constraint Description
NOT NULL Ensures that a column cannot have a NULL value.
DEFAULT Provides a default value for a column when none is specified.
UNIQUE Ensures that all the values in a column are different
PRIMARY Uniquely identifies each row/record in a database table
FOREIGN Uniquely identifies a row/record in any another database table
CHECK The CHECK constraint ensures that all values in a column satisfy certain    conditions.
INDEX Used to create and retrieve data from the database very quickly.

You can consider Primary Key constraint to be a combination of UNIQUE and NOT NULL constraint. This means that if a column is set as a primary key, then this particular column cannot have any null values present in it and also all the values present in this column must be unique.

What is Primary Key in SQL ?

Published on 14-Dec-2021 17:04:38

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