Question

Given this table :

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)

I want to select the most recent last_activity_time (standard Unix timestamp) for each user who is logged in (i.e has one or more rows where logout_time is not zer0).

I tried

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

…but that found only a single entry with two users logged in, probably because I am selecting for MAX(last_activity_time)

What I want is something like

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

all in one single SELECT statement. How can I do that?

Answer 1

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