Question

我有2个表（用户和信息）。我需要选择所有用户数据和相关的最后插入的'信息文本'（insert_time）

表用户

+----+--------+----------+
| id | name   | adress   |
+----+--------+----------+
|  1 | Name 1 | Adress 1 |
|  2 | Name 2 | Adress 2 |
|  3 | user 3 | adress 3 |
|  4 | user 4 | adress 4 |
+----+--------+----------+

表格信息

+----+---------+----------+---------------------+
| id | id_user | infotext | insert_time         |
+----+---------+----------+---------------------+
|  1 |       1 | info 1   | 2016-11-24 14:03:23 |
|  2 |       1 | info 2.  | 2016-11-24 14:08:30 |
|  3 |       3 | text 3.  | 2016-11-24 14:08:46 |
+----+---------+----------+---------------------+

我目前的查询是：

SELECT  a.*, b.infotext FROM    user a LEFT JOIN infos b
            ON a.id = b.id_user
        LEFT JOIN
        (
            SELECT      id_user, MAX(insert_time) newestInsert
            FROM        infos
            GROUP BY    id_user
        ) c ON  c.id_user = b.id_user AND
                c.newestInsert = b.insert_time

但问题是它输出的id不明显：

+----+--------+----------+----------+
| id | name   | adress   | infotext |
+----+--------+----------+----------+
|  1 | Name 1 | Adress 1 | info 1   |
|  1 | Name 1 | Adress 1 | info 2.  |
|  3 | user 3 | adress 3 | text 3.  |
|  2 | Name 2 | Adress 2 | NULL     |
|  4 | user 4 | adress 4 | NULL     |
+----+--------+----------+----------+

我需要的最终结果是：

+----+--------+----------+----------+
| id | name   | adress   | infotext |
+----+--------+----------+----------+
|  1 | Name 1 | Adress 1 | info 2.  |
|  3 | user 3 | adress 3 | text 3.  |
|  2 | Name 2 | Adress 2 | NULL     |
|  4 | user 4 | adress 4 | NULL     |
+----+--------+----------+----------+

Answer 1

将第二个条件放在on子句中。此方法将其作为相关子查询执行：

SELECT  u.*, i.infotext
FROM user u LEFT JOIN
     infos i
     ON u.id = i.id_user and
        i.insert_time = (SELECT MAX(i2.insert_time)
                         FROM infos i2
                         WHERE i2.id_user = i.id_user
                        );

Answer 2

如果表现很关键......

SELECT u.id
     , u.name   
     , u.adress   
     , i.infotext 
  FROM user u 
  LEFT 
  JOIN 
     ( SELECT x.* 
         FROM infos x 
         JOIN 
            ( SELECT id_user
                   , MAX(insert_time) insert_time 
                FROM infos 
               GROUP 
                  BY id_user
            ) y 
           ON y.id_user = x.id_user 
          AND y.insert_time = x.insert_time
     ) i 
    ON i.id_user = u.id 
 ORDER 
    BY infotext IS NULL, infotext;

MySql Left加入subselect

2 个答案: