SQL Server Primary Key

SQL Server Primary key is a single field or combination of fields that is used to uniquely define a record. Any field of a primary key cannot contain a null value. A table can have only one primary key.

You can define a primary key either in a CREATE TABLE statement or an ALTER TABLE statement.

Create Primary Key Using CREATE TABLE Statement

Syntax:

Or

Example:

Create a table "cricketers" where "cricketer_id" is a primary key.

Output:

You can verify that created table by using SELECT command:

Output:

Here cricketer_id is the primary key.

You can also create a table with primary key by using the second syntax:

Example:

Output:

You can verify that created table by using SELECT command:

Output:

Create a primary key in SQL Server where the primary key is more than one field:

Example:

Output:

You can verify that created table by using SELECT command:

Output:

Here, last_name and first_name both is primary key.

Create Primary Key Using ALTER TABLE statement

You can use an ALTER TABLE statement to create a primary key only on the column or columns that are already defined as NOT NULL.

If a column of a table has NULL values, you cannot add a primary key without dropping and recreating the table.

Syntax:

Example:

Create a primary key "cricketer_id" in the table "cricketers2".

Output:

It will create a primary key "cricketer_id" in the "cricketers2" table.