MySQL field from UNION SUBSELECT

Question

我的数据库有以下表:

• t1:博客文章
• t2:每篇博客文章的附加表
• t3:博客的英文评论
• t4:博客文章的西班牙语评论

我有以下用例:我需要获取每个博客帖子的最新评论的id连同博客帖子本身。最新的注释可能在t3或t4表中。

我想出了下面的sql，但它没有像预期的那样工作。

SELECT t1.id,
t1.one, 
t1.two, 
(
    SELECT id FROM (
        (SELECT * FROM t3 where t3.refid = t1.id) 
        UNION (SELECT * FROM t4 where t4.refid = t1.id) 
        ORDER BY datetime ASC LIMIT 1
    ) AS tempTable) 
AS someValue
FROM t1 
LEFT JOIN t2 ON (t1.id = t2.id) 
WHERE t1.otherid=42 AND t1.somefield > 0 
ORDER BY t1.someOtherField 
LIMIT 5

任何提示如果和/或如何这是可能的，高度赞赏，谢谢!

Answer 1

您需要编译一个包含t3和t4最新注释的表到表d然后将两个结果合并到注释表c中这个c将只保存一个条目，即来自t3或t4的最新id

c.t将告诉您从哪个表获得响应;

SELECT t1.*, 
       t2.*, 
       c.id AS cid, 
       c.dt AS dt, 
       c.t  AS t 
FROM   t1 
       LEFT JOIN t2 
         ON ( t1.id = t2.id ) 
       LEFT JOIN (SELECT d.id, 
                         Max(d.dt) AS dt, 
                         d.refid, 
                         d.t       AS t 
                  FROM   (SELECT t3.id, 
                                 Max(DATETIME) AS dt, 
                                 t3.refid, 
                                 3             AS t 
                          FROM   t3 
                          GROUP  BY t3.refid 
                          UNION ALL 
                          SELECT t4.id, 
                                 Max(DATETIME) AS dt, 
                                 t4.refid, 
                                 4             AS t 
                          FROM   t4 
                          GROUP  BY t4.refid) d 
                  GROUP  BY d.refid) c 
         ON c.refid = t1.id; 

Answer 2

首先尝试这个查询，它应该返回t3和t4的最新注释-

SELECT t_comments1.* FROM (SELECT * FROM t3 UNION SELECT * FROM t4) t_comments1
  JOIN (SELECT refid, MAX(datetime) max_datetime FROM (SELECT * FROM t3 UNION SELECT * FROM t4) t GROUP BY t.refid) t_comments2
    ON t_comments1.refid = t_comments2.refid AND t_comments1.datetime = t_comments2.max_datetime;

如果可以，那么让我们将此查询与t1表组合-

SELECT t1.*, t_comments.* FROM t1
  LEFT JOIN (
    SELECT t_comments1.* FROM (SELECT * FROM t3 UNION SELECT * FROM t4) t_comments1
      JOIN (SELECT refid, MAX(datetime) max_datetime FROM (SELECT * FROM t3 UNION SELECT * FROM t4) t GROUP BY t.refid) t_comments2
        ON t_comments1.refid = t_comments2.refid AND t_comments1.datetime = t_comments2.max_datetime
  ) t_comments
  ON t1.id = t_comments.refid;