从CASE MySQL中嵌套的SELECT中返回多个列

Question

有一个更好的方法吗？

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

理想情况下,我希望只有一个CASE部分从相关表中返回姓名和电子邮件.

Answer 1

我会在两个表上使用左外连接：

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

关于唯一的一种情况，如果您需要结果集上有两个不同的列，则需要两个case句子。