Table #1: qa_returns_items

Table #2: qa_returns_residues

I have a long time trying to get this Result:

item_code -   item_quantity
2         -   1
3         -   2

IF qa_returns_items.item_code = qa_returns_residues.item_code AND status_code = 11 THEN

item_quantity = qa_returns_items.item_quantity - qa_returns_residues.item_quantity

ELSEIF qa_returns_items.item_code = qa_returns_residues.item_code AND status_code = 12 THEN

item_quantity = qa_returns_items.item_quantity + qa_returns_residues.item_quantity

ELSE

show diferendes

END IF

I tried this Query:

select  SubQueryAlias.item_code, 
        total_ecuation, SubQueryAlias.item_unitprice,
        SubQueryAlias.item_unitprice * total_ecuation as item_subtotal,
        item_discount,
        (SubQueryAlias.item_unitprice * total_ecuation) - item_discount as item_total
from    (
        select  
                ri.item_code
        ,       case status_code
                when 11 then ri.item_quantity - rr.item_quantity 
                when 12 then ri.item_quantity + rr.item_quantity
                end as total_ecuation
        ,       rr.item_unitprice
        ,       rr.item_quantity
        ,       rr.item_discount * rr.item_quantity as item_discount
        from    qa_returns_residues rr
        left join    qa_returns_items ri
        on      ri.item_code = rr.item_code
        WHERE ri.returnlog_code = 1
        ) as SubQueryAlias
where   total_ecuation > 0 GROUP BY (item_code);

The query returns this result:

item_code -   item_quantity
1         -   2
2         -   2