Jag skulle förmodligen använda en UNION .
SELECT `date`
FROM `tasks` WHERE `user_id`=1
UNION
SELECT `t1`.`date`
FROM `tasks` `t1`
INNER JOIN `urls` `u1` ON `u1`.`id` = `t1`.`url_id`
WHERE `u1`.user_id`=1;
Jag skulle förmodligen använda en UNION .
SELECT `date`
FROM `tasks` WHERE `user_id`=1
UNION
SELECT `t1`.`date`
FROM `tasks` `t1`
INNER JOIN `urls` `u1` ON `u1`.`id` = `t1`.`url_id`
WHERE `u1`.user_id`=1;