Question

所以我有三个表：

Users
+-------+-----+----+
| id    | val1|val2|
+-------+-----+----+
| 1     |  1  |3   |
| 2     |  2  |5   |
| 3     |  4  |7   |
+-------+-----+----+

UsersData
+----+--------------+------------+-----|
| id | users_id     | created_at | gold|
+----+--------------+------------+-----|
|  9 |  1           |121454561212| 14  |
|  10|  1           |131454561212|  2  |
|  11|  2           |111454561212| 99  |
+----+--------------+------------+-----+

Extra
+----+------------+-----|
| id | users_id   | val4|
+----+------------+-----|
|  1 |  1         |  5  |
|  2 |  1         |  6  |
|  3 |  1         |  7  |
+----+------------+-----+

所以我希望实现（在单个查询中）是为ID = 1的用户获取单行结果，该结果满足：

用户表中的所有内容
该用户的最新条目的黄金值（users_id = 1，created_at = MAX）
Extra表中最大的val4，其中users_id = 1

因此结果行将如下所示：

+-------+-----+----+-----+----+
| id    | val1|val2|gold |val4|
+-------+-----+----+-----+----|
| 1     |  1  |3   | 2   |  7 |
------------------------------+

我可以用

完成第一部分

SELECT Users.id, Users.val1, Users.val2, UsersData.gold
FROM UsersData
LEFT JOIN Users ON UsersData.users_id = Users.id
WHERE Users.id = 1 
ORDER BY UsersData.created_at DESC
LIMIT 1

第二部分带有

SELECT MAX(Distances.distance) AS maxdistance FROM Distances WHERE Distances.users_id = 1

但是无论如何尝试，我都无法将它们组合起来……我真的很想在单个查询中完成此任务，显然我可以用多个查询完成它-但我认为这只是我缺乏mysql技能而已是这里的问题。

谢谢！

Answer 1

只需使用子查询：

SELECT Users.id, Users.val1, Users.val2, UsersData.gold,
(SELECT MAX(Distances.distance) FROM Distances WHERE Distances.users_id = Users.id) AS maxdistance
FROM UsersData
RIGHT JOIN Users ON UsersData.users_id = Users.id
WHERE Users.id = 1 
ORDER BY UsersData.created_at DESC
LIMIT 1

这是由Users.id连接的子查询：

SELECT MAX(Distances.distance) FROM Distances WHERE Distances.users_id = Users.id) AS maxdistance

Answer 2

我将使用以下子查询：

select u.*,
       (select ud.gold
        from userdata ud
        where ud.users_id = u.id
        order by ud.created_at desc
        limit 1
       ) as most_recent_gold,
       (select max(e.val4)
        from extra e
        where e.users_id = u.id
       ) as max_val4
from users u
where u.id = 1 ;

mysql LEFT JOIN主表和另外两个表，每个表都有最大值

2 个答案: