Creating UNIQUE Constraints on Existing Tables : Unique « Constraints « SQL Server / T-SQL Tutorial

4>
5>
6>    CREATE TABLE Employees
7>    (
8>       EmployeeID       int           IDENTITY  NOT NULL,
9>       FirstName        varchar(25)             NOT NULL,
10>       MiddleInitial    char(1)                 NULL,
11>       LastName         varchar(25)             NOT NULL,
12>       Title            varchar(25)             NOT NULL,
13>       SSN              varchar(11)             NOT NULL,
14>       Salary           money                   NOT NULL,
15>       PriorSalary      money                   NOT NULL,
16>       LastRaise AS Salary - PriorSalary,
17>       HireDate         smalldatetime           NOT NULL,
18>       TerminationDate  smalldatetime           NULL,
19>       ManagerEmpID     int                     NOT NULL,
20>       Department       varchar(25)             NOT NULL
21>    )
22>    GO
1>
2>    ALTER TABLE Employees
3>       ADD CONSTRAINT AK_EmployeeSSN
4>       UNIQUE (SSN)
5>    GO
1>
2>    drop table Employees;
3>    GO

7.10.Unique
	7.10.1.	Adding a UNIQUE Constraint to an Existing Table
	7.10.2.	Unique constraints will also allow one NULL, whereas primary keys will not.
	7.10.3.	Unique Constraint: Enforce uniqueness on a column which is not the column used to primarily identify each row.
	7.10.4.	Unique constraints can be created when initially creating the table or added after.
	7.10.5.	Unique constraint
	7.10.6.	Creating a UNIQUE Constraints when creating a table
	7.10.7.	Creating UNIQUE Constraints on Existing Tables