How to write a procedure for updating a table .

Question

I had a table like this

id int identitty(1,1)
productname varchar(20)
Status char(1) like this

IF(@ACTION='U')

BEGIN

IF EXISTS(SELECT P_ID FROM TBL_PRODUCTS WHERE P_ID=@P_ID) 

BEGIN

 set @exp=(select COUNT(*) from TBL_PRODUCTS where P_NAME=@P_NAME)

 set @exp1=(select count(*) from tbl_products where p_id=@P_ID and p_name=@P_NAME)

 if((@exp<=1)and (@exp1=1))

   Begin

UPDATE TBL_PRODUCTS SET

P_NAME=@P_NAME,

P_TAG_LINE=@P_TAG_LINE,

P_WEB=@P_WEB,

P_ACTIVE=@P_ACTIVE

WHERE P_ID=@P_ID;

IF(@@ERROR=0)

BEGIN

SET @ERROR_ID=1

END

ELSE

BEGIN

SET @ERROR_ID=0

END

End

ELSE

 BEGIN

  SET @ERROR_ID=-2

  END

END

ELSE

BEGIN

SET @ERROR_ID=-2

END

END


here i can able to Update a record if the id is same and name is same and i want to do 

product name should not repeat twice in table..