PRIMARY KEY in SQL

PRIMARY KEY is a combination UNIQUE and NOT NULL which does not allow the duplicate and NULL values in to columns on which it is imposed or applied.

Table can be imposed with any number of UNIQUE or NOT NULL constraints but can have only a single PRIMARY KEY constraint which can be imposed either on a single column or multiple columns (COMPOSITE PRIMARY KEY).

As per specification of Dr.F.C.Codd, a table can have only one key or identity value for uniquely identifying or picking the complete row. The column or columns on which you imposed the PRIMARY KEY will be considered as identity of your table.

Example:

Create Table BANK (CustomerId Int PRIMARY KEY, Cname Varchar (50), Balance Decimal (7,2) NOT NULL)

We can also give a constraint name at column or table level.

Example-constraint name at column level:

Create Table BANK (CustomerId Int CONSTRAINT CID_PK PRIMARY KEY, Cname Varchar (50), Balance Decimal (7, 2) NOT NULL)

Example-constraint name at table level:

Create Table BANK (CustomerId Int, Cname Varchar (50), Balance Decimal (7, 2) NOT NULL, CONSTRAINT CID_PK PRIMARY KEY)