尽管主记录为空，LEFT JOIN应该如何返回值？

Question

我的SQL查询是这样的：

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 

当total不NULL为orderID 10将返回sTax价值。如果total是NULL对于orderID 10sTax是NULL。

如何获得sTax虽然如果total是NULL？

Answer 1

这里查询你正在寻找：

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 

查询返回的每个项目的总价格，如果这个总数NULL，返回税（否则税是NULL）。

希望这会有所帮助。