UNION SUBSELECT 中的 MySQL 字段

Question

我的数据库有以下表格：

• t1：博客文章
• t2：每个博客文章的附加表
• t3：博客文章的英语评论
• 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文章的最新评论的 ID 以及博客文章本身。最新的评论可能位于 t3 或 t4 表中。

我想出了以下sql，但它没有按预期工作。

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

任何有关是否和/或如何可能的提示都将受到高度赞赏，谢谢！

Answer 1

您需要将包含 t3 和 t4 的最新评论的表编译到表 d 中
然后将两个结果合并到注释表c中
此 c 仅保存 1 个条目，即来自 t3 或 t4 的最新 ID，

ct 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 的最新评论 -

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 表结合起来 -

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;