Sep
Primary Key:
Primary Key enforces uniqueness of the column on which they are defined. Primary Key creates a clustered index on the column. Primary Key does not allow Nulls.
Create table with Primary Key:
CREATE TABLE Authors (
AuthorID INT NOT NULL PRIMARY KEY,
Name VARCHAR(100) NOT NULL
)
GO
Alter table with Primary Key:
ALTER TABLE Authors
ADD CONSTRAINT pk_authors PRIMARY KEY (AuthorID)
GO
Unique Key:
Unique Key enforces uniqueness of the column on which they are defined. Unique Key creates a non-clustered index on the column. Unique Key allows only one NULL Value.
Alter table to add unique constraint to column:
ALTER TABLE Authors ADD CONSTRAINT IX_Authors_Name UNIQUE(Name)
GO
Reference :http://blog.SQLAuthority.com
see this also to – http://blog.sqlauthority.com/2008/09/08/sql-server-%E2%80%93-2008-creating-primary-key-foreign-key-and-default-constraint/
Tags: Constraints, Primary key, SQL Server, Unique key
One Response to "SQL SERVER–Primary Key Constraints and Unique Key Constraints"
October 21st, 2009 at 1:19 am
[...] SQL SERVER–Primary Key Constraints and Unique Key Constraints Primary Key: Primary Key enforces uniqueness of the column on which they are defined. Primary Key creates a clustered index on the column. Primary Key does not allow Nulls. Create table with Primary Key: CREATE TABLE Authors ( AuthorID INT NOT NULL PRIMARY KEY, Name VARCHAR(100) NOT NULL ) GO Alter table with Primary Key: ALTER TABLE Authors ADD CONSTRAINT … [...]