Get stock from Purchased and Sold Products QTY

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VB

SQL

I have been working on Client project. I need to get stock from Purchased product QTY and Sold QTY.

Here my SQL

SELECT DISTINCTROW tbl_itemmaster.product_code,tbl_itemmaster.unittype,tbl_itemmaster.color,SUM(tbl_purchasedetails.qty),SUM(tbl_saledetails.qty),(SUM(tbl_purchasedetails.qty)-SUM(tbl_saledetails.qty)) 
FROM (tbl_itemmaster LEFT JOIN tbl_saledetails ON tbl_itemmaster.[product_code] = tbl_saledetails.[product_code]) 
LEFT JOIN tbl_purchasedetails ON tbl_itemmaster.[product_code]=tbl_purchasedetails.[product_code] GROUP BY tbl_itemmaster.product_code,tbl_itemmaster.unittype,tbl_itemmaster.color

Below is the result of sql

-----------------------------------------------------------------
Product Code   Unit Type  Color     Purchase QTY  Sold QTY  Stock
-----------------------------------------------------------------
1001           KiloGram   Red       500           
500            Kilogram   White     1033          300       733
570            Kilogram   Black   
600            Kilogram   Pink   
9005           Kilogram   Magenta   800.5         
900            Kilogram   Green   
-----------------------------------------------------------------

Product code 500 has three times purchase entry i.e. 500+400+133 = 1033 Purchased Qty
Product code 500 has only ONE time sale entry i.e. 100 = 100 Sold Qty
Product code 500 suppose to has 933 Stock Qty
BUT why I getting 300 in sold qty ?
Where my query is lacking?
PLEASE HELP !!!