SELECT contests.id, contests.post_id, contests.status, GROUP_CONCAT(DISTINCT members.uid ORDER BY members.uid ASC SEPARATOR ', ') AS members
FROM `contests` contests
LEFT JOIN `members` members ON contests.post_id=members.post_id
GROUP BY contests.post_id;

SELECT contests.id, contests.post_id, contests.status, GROUP_CONCAT(DISTINCT members.uid ORDER BY members.uid ASC SEPARATOR ', ') AS members
FROM `contests` contests
WHERE members.won = 1 AND contests.status = 1
LEFT JOIN `members` members ON contests.post_id=members.post_id
GROUP BY contests.post_id;

Unexpected ordering of clauses. (near "WHERE" at position 168)