Question

鉴于此表：

mysql> describe activity;
+---------------------------+-------------+------+-----+---------+-------+
| Field                     | Type        | Null | Key | Default | Extra |
+---------------------------+-------------+------+-----+---------+-------+
| user_id                   | varchar(16) | NO   |     | NULL    |       |
| login_time                | int(11)     | NO   |     | NULL    |       |
| last_activity_time        | int(11)     | NO   |     | NULL    |       |
| last_activity_description | text        | YES  |     | NULL    |       |
| logout_time               | int(11)     | NO   |     | NULL    |       |
+---------------------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

我想为每个登录的用户选择最新的last_activity_time（标准Unix时间戳）（即有一行或多行logout_time不是zer0）。

我试过

SELECT user_id, login_time, MAX(last_activity_time) 
  FROM activity  
 WHERE logout_time="0";

...但是只找到一个有两个用户登录的条目，可能是因为我选择了MAX(last_activity_time)

我想要的是像

SELECT all unique user_ids  
  SELECT each of those which has one or more entries where `logout_time` != 0  
     SELECT the maximum value of `logout_time` for each of those

所有在一个SELECT语句中。我怎样才能做到这一点？

Answer 1

SELECT user_id, MAX(logout_time)
FROM activity 
WHERE logout_time <> "0" 
GROUP BY user_id;

如何编写这个SELECT语句？

1 个答案: