Combined MySQL query help - 2 tables - mysql

Question

I have a table rosters and a table members. They are setup like this:
rosters table
id team member
1 1 1
2 1 2
3 1 3
members table
id name last_active
1 Dan 1454815000
2 Ewan 1454817500
3 Cate 1454818369
I need to fetch all rows in rosters where team=1. Then, I need to take all those returned results and use the member column to fetch the largest last_active value from members where id is in that list of results returned from rosters
This is how I would do it in PHP, but I'm sure there's a way to just use a more efficient query.
$rosterList = $db->query('SELECT * FROM rosters WHERE team=1');
$lastActive = 0;
foreach($rosterList as $roster) {
$activity = $db->query('SELECT last_active FROM members WHERE id='.$roster['team']);
if ( $activity > $lastActive )
$lastActive = $activity;
}
if ( $lastActive > time()-60 )
echo 'team is currently online';
It would be nice if it could just return one result with the latest last_active column but if it returns all matches in the members table that would be fine too.

Answer 1

By using an ORDER BY (descending) on that last_actie column, then limiting to just 1 row, you get access to the whole member row.
sqlfiddle demo
MySQL 5.6 Schema Setup:
CREATE TABLE members
(`id` int, `name` varchar(4), `last_active` int)
;
INSERT INTO members
(`id`, `name`, `last_active`)
VALUES
(1, 'Dan', 1454815000),
(2, 'Ewan', 1454817500),
(3, 'Cate', 1454818369)
;
CREATE TABLE rosters
(`id` int, `team` int, `member` int)
;
INSERT INTO rosters
(`id`, `team`, `member`)
VALUES
(1, 1, 1),
(2, 1, 2),
(3, 1, 3)
;
Query 1:
select
m.*
from members m
join rosters r on m.id = r.member
where r.team = 1
order by m.last_active DESC
limit 1
Results:
| id | name | last_active |
|----|------|-------------|
| 3 | Cate | 1454818369 |

Answer 2

You can use following solution:
$result_array = mysql_query("SELECT * FROM rosters as r INNER JOIN members as m on r.member=m.id AND r.team = '1' ORDER BY last_active DESC LIMIT 1");
$lastActive = 0;
if($result_array)
{
while($row = mysql_fetch_row($result_array,MYSQL_ASSOC))
{
$lastActive = $result_array['last_active'];
}
}
if ( !empty($lastActive) && $lastActive > time()-60 )
echo 'team is currently online';

Answer 3

I'm a little confused why you are retrieving rows from members where the id value matches the team value from rosters. Seems like you would want to match on the member column.
You can use a join operation and an aggregate function. To get the largest value of last_active for all members of a given team, using the member column, something like this:
SELECT MAX(m.last_active) AS last_active
FROM members m
JOIN rosters r
ON r.member = m.id
WHERE r.team = 1
To do the equivalent of the original example, using the team column (and again, I don't understand why you would do this, because it doesn't look right):
SELECT MAX(m.last_active) AS last_active
FROM members m
JOIN rosters r
ON r.team = m.id
WHERE r.team = 1
The MAX() aggregate function in the SELECT list, with no GROUP BY clause, causes all of the rows returned to be "collapsed" into a single row. The largest value of last_active from the rows that satisfy the predicates will be returned.
You can see how the join operation works by eliminating the MAX() aggregate...
SELECT m.last_active
, m.id AS member_id
, m.name AS member_name
, r.member
, r.team
, r.id AS roster_id
FROM members m
JOIN rosters r
ON r.member = m.id
WHERE r.team = 1
ORDER BY m.last_active DESC