I have always enjoyed writing about three topics Constraint and Keys, Backup and Restore and Datetime Functions.
Primary Keys constraints prevents duplicate values for columns and provides unique identifier to each column, as well it creates clustered index on the columns.
-- Primary Key Constraint upon Table Created Method 1
USE AdventureWorks
GO
CREATE TABLE ConstraintTable
(ID INT CONSTRAINT Ct_ID PRIMARY KEY, ColSecond INT)
GO
--Clean Up
DROP TABLE ConstraintTable
GO
-- Primary Key Constraint upon Table Created Method 2
USE AdventureWorks
GO
CREATE TABLE ConstraintTable
(ID INT, ColSecond INT,
CONSTRAINT Ct_ID PRIMARY KEY (ID))
GO
--Clean Up
DROP TABLE ConstraintTable
GO
-- Primary Key Constraint after Table Created Method 3
USE AdventureWorks
GO
CREATE TABLE ConstraintTable
(ID INT, ColSecond INT)
GO
-- Primary Key Column must not allow NULL
ALTER TABLE ConstraintTable
ALTER COLUMN ID INT NOT NULL
GO
ALTER TABLE ConstraintTable
ADD CONSTRAINT Ct_ID PRIMARY KEY (ID)
--Clean Up
DROP TABLE ConstraintTable
GO
Reference : Pinal Dave (https://darkslategrey-bat-805937.hostingersite.com)
