UNIQUE key in SQL

If UNIQUE constraint is imposed on a column or column’s the column will not allow duplicate values in to it.

Syntax:

Create Table <Table_name> <column 1><data type> [width] [UNIQUE],… <column N><data type> [width] [UNIQUE]

Example:

Create Table BANK (Customerid Int UNIQUE, Cname varchar (50), Balance Decimal (7,2) NOT NULL)

Note:

a) When a UNIQUE constraint is imposed on a column the column will not allow duplicate value but allow NULL value.

b) UNIQUE constraint in MS-SQL Server allows to store single NULL value in it but Oracle allows multiple NULL values.

UNIQUE constraint can be applied on multiple columns combined or separately also, NOT NULL can be applied on multiple columns separately.

UNIQUE key on multiple column's Example:

Create Table BranchDetails (Citycode varchar (10), Branchcode varchar (10), Location varchar (20), constraint CC_BC_UQ UNIQUE (Citycode, Branchcode)