Select Most Recent Items with dup values

Given the following information, how can I select the most recent line items (based on time_entered) on unique params and cron_action_id pairs that haven't been executed?

cron_s开发者_如何转开发chedule

For example, ids 1, 2, and 4 have the same params and cron_action_id, so I need not select all 3, just id 4. Same principle for id 3/5 and 7/8.

I can only get so far as

SELECT *
FROM cron_schedule cs
INNER JOIN cron_actions ca
ON cs.cron_action_id = ca.cron_action_id
WHERE time_executed = 0
-- GROUP BY (params, cron_action_id) ?

This should return rows with id 4, 5, 6, and 8

Thanks

---

SELECT t1.* 
FROM   cron_schedule t1 
  INNER JOIN (SELECT params, 
    cs.cron_action_id, 
    Max(time_entered) AS time_entered 
    FROM cron_schedule cs 
    INNER JOIN cron_actions ca 
    ON cs.cron_action_id = ca.cron_action_id 
    WHERE  time_executed = 0 
    GROUP  BY params, cron_action_id) AS t2 
  ON t1.params = t2.params 
    AND t1.cron_action_id = t2.cron_action_id 
    AND t1.time_entered = t2.time_entered 
  INNER JOIN cron_actions ca2 
  ON t1.cron_action_id = ca2.cron_action_id 
  WHERE  t1.time_executed = 0 

---

Not tested, but this basic idea should work:

SELECT *
FROM
    cron_schedule outer_cron_schedule JOIN cron_actions
WHERE
    time_entered = (
        SELECT MAX(time_entered)
        FROM cron_schedule inner_cron_schedule
        WHERE
            outer_cron_schedule.id = inner_cron_schedule.id
        GROUP BY
            params, cron_action_id
    )
    AND time_executed = 0

Give it a try