3 Different Methods to Create Primary Keys

Primary key is unique identifier for each record in the database table and creates clustered index on the column on which its defined.

--Cleanup
IF EXISTS ( SELECT OBJECT_ID('constraintTable'))
DROP TABLE constraintTable
GO

CREATE TABLE constraintTable
(ID INT CONSTRAINT ct_ID PRIMARY KEY, Col2 VARCHAR(20))
GO

--Cleanup
IF EXISTS ( SELECT OBJECT_ID('constraintTable'))
DROP TABLE constraintTable
GO

--Primary Key upon table creating Method2
CREATE TABLE constraintTable
(ID INT, Col2 VARCHAR(20) CONSTRAINT ct_ID PRIMARY KEY)
GO

--Cleanup
IF EXISTS(SELECT OBJECT_ID('constraintTable'))
DROP  TABLE constraintTable
GO

--Primary Key upon table creating Method3
CREATE TABLE constraintTable
(ID INT, Col2 VARCHAR(20))
GO

--Primay key doesn't allow NULL values
ALTER TABLE constraintTable
ALTER COLUMN ID INT NOT NULL
GO

ALTER TABLE constraintTable
ADD CONSTRAINT Ct_ID PRIMARY KEY (ID)
GO

Reference: DP (http://dptechnicalblog.blogspot.com/ )