Sql Query Error

The problem with your query was the wrong WHERE clause placement and not aliasing the subqueries output. I hope this will help

SELECT  Product_Code
,       ISNULL(Opening_stock, 0) AS OpeStock
FROM    Opening_Stock O
       LEFT JOIN ( SELECT  Product_Code
              ,       ISNULL(SUM(Quantity), 0) RQ
              FROM    Grn_Detail R
              WHERE   Product_Code = 2
                   AND Grn_Date <= '08/02/2012'
            ) R
       ON R.Product_Code = O.ProductCode
       LEFT JOIN ( SELECT  Product_Code
              ,       ISNULL(SUM(Quantity), 0) Iq
              FROM    Issue_Detail I
              WHERE   Product_Code = 2
                   AND Slip_Date <= '08/02/2012'
            ) I
       ON I.Product_code = O.Product_Code
WHERE   O.Product_code = 2 

I personally like @Shawn Melton's coding approach as that is a much readable and easier format. But one cannot specify any filter in the main WHERE clause for the tables used in LEFT/RIGHT JOIN other than the table for which all rows to be returned. This would turn the LEFT/RIGHT JOIN to INNER JOIN functionality.

I am no expert on T-SQL and my answer could very well be completely wrong. (Which if it is this will be a learning lesson for the both of us.)

This is just a best guess...

 SELECT  o.Product_Code  , OpeStock = ISNULL(o.Opening_stock,0)  , RQ = ISNULL(SUM(r.Quantity),0)  , IQ = ISNULL(SUM(i.Quantity),0) From Opening_Stock AS o  LEFT JOIN Grn_Detail AS r ON r.Product_Code = o.Product_Code  LEFT JOIN Issue_Detail AS i ON i.Product_Code = o.Product_Code Where o.Product_code=2  AND r.Grn_Date <= '08/02/2012'  AND i.Slip_Date <= '08/02/2012' GROUP BY o.ProductCode