Below list is major difference between Primary Key and Unique
Key Datatype in Sql Server;
|
|
Primary Key
|
Unique Key
|
|
Declare Limit
|
Only one columns can be defined in a table
|
Table can have more than one Unique Key
|
|
Indexing
|
Primary key defined as in clustered
index
|
Unique Key defined as unique Clustered index.
|
|
Nullable
|
It not allows the Null value.
|
It allow null value but only one.
|
|
Consist Of
|
PRIMARY KEY = UNIQUE KEY + Not Null CONSTRAINT
|
UNIQUE KEY
|
|
Syntax
|
e.g.
a single column PRIMARY KEY column
in table;
Create Syntax for primary Key:
CREATE TABLE dbo.employee
(
EmpId INT NOT NULL PRIMARY KEY,
Name VARCHAR(50),
Address VARCHAR(100),
City VARCHAR(50)
)
Alter Syntax for primary Key:
ALTER TABLE dbo.employee
DROP CONSTRAINT PK_EMPLOYEE
|
e.g.
single column UNIQUE KEY column while
creating a table
Create Syntax Unique Key;
CREATE TABLE dbo.employee
(
EmpId INT NOT NULL UNIQUE ,
Name VARCHAR(50),
Address VARCHAR(100),
City VARCHAR(50)
)
Alter Syntax Unique Key;
ALTER TABLE dbo.employee
ADD CONSTRAINT UK_EMPLOYEE UNIQUE (EmpId)
|
No comments:
Post a Comment