We will discuss about the SQL
unique constraint and difference between Primary Key and Unique key and demonstrate
how to create/drop unique constraint
The SQL unique constraint ensures
that all values in a columns are unique.
By default Primary Key constraint
also granted the uniqueness of columns values and the only difference is that
primary key does not accept NULL value but unique key constraint accept a
single NULL value in column.
Difference
between Primary Key Vs Unique Key:
·
Only one Primary key per table but we can create multiple unique
key per table.
Here is a table OrderStatus,
which contains Primary Key on StatusID
Create Table OrderStatus
(
StatusID int IDENTITY(1,1) NOT NULL,
StatusCode varchar(3) NOT NULL,
StatusDescription varchar(50) NOT NULL,
CONSTRAINT [OrderStatus_PK] PRIMARY KEY CLUSTERED ( StatusID ASC)
)
Syntax: Create
SQL Unique Constraint on Create Table:
The below SQL script will create
a unique constraint ‘OrderStatusCode_UC ’ on a Column StatusCode in table and it ensures
that Column should not duplicate Code.
Create Table OrderStatus
(
StatusID int IDENTITY(1,1) NOT NULL,
StatusCode varchar(3) NOT NULL,
StatusDescription varchar(50) NOT NULL,
CONSTRAINT [OrderStatus_PK] PRIMARY KEY CLUSTERED ( StatusID ASC),
CONSTRAINT OrderStatusCode_UC UNIQUE (StatusCode)
)
Syntax: Create
SQL Unique Constraint on Alter Table:
The below SQL script will create
a unique constraint ‘OrderStatusCode_UC’ on
already existing table
ALTER TABLE OrderStatus
ADD CONSTRAINT OrderStatusCode_UC UNIQUE (StatusCode);
The below SQL script will create
a unique constraint ‘OrderStatusDescCode _UC’ on multiple columns
ALTER TABLE OrderStatus
ADD CONSTRAINT OrderStatusDescCode_UC UNIQUE (StatusCode, StatusDescription);
Drop SQL
Unique Key Constraint:
You can simply use below SQL
script to Drop unique constraint
ALTER TABLE OrderStatus
DROP CONSTRAINT OrderStatusDescCode_UC;
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