從SELECT中返回嵌套在SELECT中的多列MySQL

Question

有沒有更好的方法來做到這一點？

SELECT subs. * , 
     CASE subs.member_type 
     WHEN 'member' THEN 
     (SELECT CONCAT_WS(' ', members.first_name, members.last_name) 
      FROM members 
      WHERE members.id = subs.member_id) 
     ELSE 
     (SELECT members_anon.username 
      FROM members_anon 
      WHERE members_anon.id = subs.member_id) 
     END AS fullname, 
     CASE subs.member_type 
     WHEN 'member' THEN 
     (SELECT members.email 
      FROM members 
      WHERE members.id = subs.member_id) 
     ELSE 
     (SELECT members_anon.email 
      FROM members_anon 
      WHERE members_anon.id = subs.member_id) 
     END AS email 
    FROM subs 
WHERE subs.item_id =19 
    AND subs.item_type = 'blog' 
LIMIT 0 , 30 

理想情況下，我想只有一個CASE從相關表中返回的姓名和電子郵件部分。

Answer 1

我會用左外連接兩個表：

SELECT subs. * , 
     CASE subs.member_type 
     WHEN 'member' THEN CONCAT_WS(' ', m.first_name, m.last_name) 
     ELSE ma.username 
     END AS fullname, 
     CASE subs.member_type 
     WHEN 'member' THEN m.email 
     ELSE ma.email 
     END AS email 
    FROM subs 
    LEFT OUTER JOIN members m on (m.id = subs.member_id) 
    LEFT OUTER JOIN members_anon ma on (ma.id = subs.member_id) 
WHERE subs.item_id =19 
    AND subs.item_type = 'blog' 
LIMIT 0 , 30 

關於僅有一例心願，如果你需要在你的結果集兩個不同的列，您將需要兩個case句子。

Answer 2

不能使用一個單一的情況下，表達式處理兩個單獨的列...

用途：

SELECT s. *, 
      CASE s.member_type 
      WHEN 'member' THEN x.fullname 
      ELSE y.fullname 
      END AS fullname, 
      CASE subs.member_type 
      WHEN 'member' THEN x.email 
      ELSE y.email 
      END AS email 
    FROM SUBS s 
LEFT JOIN (SELECT m.id, 
        CONCAT_WS(' ', members.first_name, members.last_name) AS fullname, 
        m.email 
      FROM MEMBERS m) x ON x.id = s.member_id 
LEFT JOIN (SELECT ma.id, 
        ma.username, 
        ma.email 
      FROM MEMBERS_ANON ma) y ON y.id = s.member_id       
    WHERE s.item_id = 19 
     AND s.item_type = 'blog' 
    LIMIT 0 , 30