儘管主記錄爲空，LEFT JOIN應該如何返回值？

Question

我的SQL查詢是這樣的：

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 

當total不NULL爲orderID 10將返回sTax價值。如果total是NULL對於orderID 10sTax是NULL。

如何獲得sTax雖然如果total是NULL？

Answer 1

這裏查詢你正在尋找：

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 

查詢返回的每個項目的總價格，如果這個總數NULL，返回稅（否則稅是NULL）。

希望這會有所幫助。