MySQL：使用別名作為列

Question

我有這個問題：

select qa_returns_items.item_code, 
               (CASE status_code
                 WHEN 11 THEN (qa_returns_items.item_quantity - qa_returns_residues.item_quantity) 
                 WHEN 12 THEN (qa_returns_items.item_quantity + qa_returns_residues.item_quantity)   END) as total_ecuation , 
                qa_returns_residues.item_unitprice,
               ( qa_returns_residues.item_unitprice * total_ecuation) as item_subtotal,
               (qa_returns_residues.item_discount * item_quantity) as item_discount,
               ( ( qa_returns_residues.item_unitprice * total_ecuation) -   
                 (qa_returns_residues.item_discount * item_quantity) ) as item_total
from qa_returns_residues, qa_returns_items
where
    total_ecuation > 0 
AND qa_returns_items.item_code = qa_returns_residues.item_code;

它向我顯示錯誤： '字段列表'中的未知列'total_ecuation'

我如何將別名用作列？

Answer 1

考慮使用子查詢。 我添加了表別名以使查詢更具可讀性：

select  *
,       item_unitprice * total_ecuation as item_subtotal
,       (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
        join    qa_returns_items ri
        on      ri.item_code = rr.item_code
        ) as SubQueryAlias
where   total_ecuation > 0

Answer 2

我擔心您必須重寫查詢以使用命名子查詢，或者在where子句中重復整個case語句...