Unique key and default constraint

We use UNIQUE constraint to enforce uniqueness of a column.

Column shouldn't allow any duplicate values.

 

Create table

 

create table Employee

(

EmployeeId int,

Username varchar(100)

)

 

Alter table to add unique key

 

Alter Table Employee

Add Constraint uk_uname Unique(Username)

 

Alter table to drop unique key

 

Alter Table Employee

Drop COnstraint uk_uname

The DEFAULT constraint is used to insert a default value into a column.

The default value will be added to all new records, if no other value is specified.

create table Employee

(

EmployeeId int,

Name varchar(50),

age int,

salary money

)

ALTER TABLE Employee

ADD CONSTRAINT df_emp_sal default 0.0

ALTER TABLE Employee

drop CONSTRAINT df_emp_sal