Question

I was told to rewrite thie following query with a nested query without joins for better performance. Is it true? If so, how to rewrite it?

SELECT distinct A.Company_Name,C.Outlet_Name,
       C.Outlet_FCE_ID,D.Usergroup_Name
FROM   company A, PURCHASE_INVOICE B, 
       Outlet C, User_Group D,CT_USER E
WHERE A.Company_ID = B.Company_ID AND B.Outlet_ID  = C.Outlet_ID 
  AND B.Company_ID = C.Company_ID AND B.Username   = E.Username 
  AND E.Usergroup_ID=D.Usergroup_ID

Here is the table structure.

Answer 1

Let me answer my own question. I don’t like it this way, but here we go….

    SELECT  distinct
        ( SELECT    Company_Name
          FROM      dbo.COMPANY
          WHERE     Company_ID = p.Company_ID
        ) AS 'CompanyName' ,
        ( SELECT    Outlet_Name
          FROM      dbo.OUTLET
          WHERE     Company_ID = p.Company_ID
                    AND Outlet_ID = p.Outlet_ID
        ) AS 'OutletName' ,
        ( SELECT    Outlet_FCE_ID
          FROM      dbo.OUTLET
          WHERE     Company_ID = p.Company_ID
                    AND Outlet_ID = p.Outlet_ID
        ) 'OutletFCEID' ,
        ( SELECT    Usergroup_Name
          FROM      dbo.USER_GROUP
          WHERE     Usergroup_ID IN ( SELECT    Usergroup_ID
                                      FROM      CT_USER
                                      WHERE     Username = p.UserName )
        ) 'UsergroupName'
FROM    dbo.PURCHASE_INVOICE p