Question

我的SQL查询就像：

SELECT SUM(i.itemPrice) as total, t.sTax 
FROM items AS i 
LEFT JOIN tax_table AS t ON t.branchID = 10 
WHERE i.orderID = 10

<{>} total 的NULL不是orderID 10时，它将返回sTax值。如果<{>} total 的NULL为orderID 10，则sTax为NULL。

如果sTax是total，我该如何获得NULL？

Answer 1

这是您正在寻找的查询：

SELECT SUM(i.itemPrice) AS total 
     ,CASE
        WHEN SUM(i.itemPrice) IS NULL THEN (SELECT t.sTax
                                            FROM tax_table AS t
                                            WHERE t.branchID = 10
                                            LIMIT 1)
        ELSE NULL
       END AS tax
FROM items AS i
WHERE i.orderID = 10
GROUP BY i.orderID

查询会返回每件商品的总价格，如果此总额为NULL，则会退回税款（否则税额为NULL）。

希望这会有所帮助。

LEFT JOIN如何返回值虽然主记录为空？

1 个答案: