sql server modify constraints

--Example 1
 ALTER TABLE stuInfo ADD CONSTRAINT PK_stuNo PRIMAR ... --Example
 1 ALTER
 TABLE  stuInfo  ADD CONSTRAINT  PK_stuNo  PRIMARY KEY  (stuNo)


--Example 2   Add a primary key constraint ( Primary Key )
 --There is a primary key constraint PK_stuNO , then delete IF
 EXISTS ( SELECT * FROM sysobjects  WHERE  name = ' PK_stuNo' and  xtype= 'PK' )
   Alter TABLE  stuInfo
     Drop Constraint  PK_stuNo
 Go
 --Re- Add primary key constraint PK_stuNO
 ALTER TABLE  stuInfo   ADD CONSTRAINT  PK_stuNo  PRIMARY KEY  (stuNo)
 Go --Example
 3 Add unique UQ     
Constraint ( Unique Constraint )
 -- there is a unique constraint UQ_stuNO , then delete
 IF  EXISTS ( SELECT  *  FROM  sysobjects  WHERE  name = 'UQ_stuID'  and  xtype= 'UQ' )
   Alter TABLE  stuInfo
     Drop Constraint  UQ_stuID
 Go
 -- Re-add the unique constraint UQ_stuID
 ALTER TABLE  stuInfo  ADD CONSTRAINT  UQ_stuID  UNIQUE  (stuID)

--Example 4 Add default DF constraint ( Default Constraint )
 -- If there is default constraint UQ_stuNO , drop
 IF EXISTS ( SELECT * FROM  sysobjects  WHERE name = 'DF_stuAddress' and  xtype= 'D' )
   Alter TABLE  stuInfo  Drop Constraint  DF_stuAddress
 Go
 -- Re-add default constraint DF_stuAddress
 ALTER TABLE  stuInfo   ADD CONSTRAINT  DF_stuAddress  DEFAULT  ( ' Address unknown '     ) FOR stuAddress

--Example 5 Check CK Constraint ( Check Constraint )
 -- Check Constraint UQ_stuNO exists , then delete
 IF EXISTS ( SELECT * FROM  sysobjects  WHERE name = 'CK_stuAge' and  xtype= 'C' )
   Alter TABLE  stuInfo  Drop  Cons