MySQL的條件加入使用同一個表

Question

我想做的使用LEFT JOIN（請不要建議UNION ALL）以下

SELECT o.*, s.col1, s.col2 FROM order o 
    INNER JOIN user u ON o.user_id = u.id 

    IF o.date less than '2011-01-01' 
     JOIN subscribe s ON u.key = s.key 
    ELSE 
     JOIN subscribe s ON u.email = s.email 
    END IF; 

我用下面的，但不能對其進行測試。

SELECT o.*, COALESCE(s1.col1,s2.col1) AS 
    col1, COALESCE(s1.col2, s2.col2) AS col2 
    FROM order o INNER JOIN user u ON o.user_id = u.id 
    LEFT JOIN subscribe s1 ON 
    (u.key LIKE (CASE o.date >= '2011-01-01 00:00:00' 
         WHEN TRUE THEN s1.key ELSE NULL END)) 
     LEFT JOIN subscribe s2 ON (u.email LIKE (CASE o.date < 
     '2011-01-01 00:00:00' WHEN TRUE THEN s.email 
     ELSE NULL END)); 

如果我錯了，請糾正我。

謝謝。

Answer 1

你並不需要加入兩次，像這樣做：

SELECT o.*, s.col1, s.col2 
    FROM order o INNER JOIN user u ON o.user_id = u.id 
    LEFT JOIN subscribe s ON((o.date < '2011-01-01' AND u.key = s.key) OR 
          (o.date >= '2011-01-01' AND u.email = s.email)) 

---

這裏是不會做一個全表掃描，如果你有subscribe.key和subscribe.email指標的解決方案：

SELECT * FROM 
(
    (SELECT 0 AS mode, o.date AS odate, o.*, s.col1, s.col2 
     FROM order o INNER JOIN user u ON o.user_id = u.id 
     LEFT JOIN subscribe s ON(u.key = s.key)) 
    UNION 
    (SELECT 1 AS mode, o.date AS odate, o.*, s.col1, s.col2 
     FROM order o INNER JOIN user u ON o.user_id = u.id 
     LEFT JOIN subscribe s ON(u.email = s.email)) 
) 
WHERE (odate < '2011-01-01' AND mode = 0) OR 
     (odate >= '2011-01-01' AND mode = 1)