SQL选择以消除重复并返回子表上记录的最新日期

Question

I am trying to retrieve data from a primary table, my_form, and some from a table linked via a linktable. There is a possibility that the actions table could contain more than one action_required = 'Close' record for the header record.

Here are my three tables:

my_form:
# idmy_form, summary, description, station
'1', 'OSR puncture', 'Puncture while driving to work', 'Thornbury'
'2', 'Windscreen wiper broke', 'Wiper broke while going shopping', 'Bristol'
'3', 'Out of screenwash', 'Run out of screen wash en route to Cardiff', 'Cardiff'


form_action:
# idmy_form, action_id
'1', '201'
'1', '202'
'1', '203'
'2', '301'
'2', '302'
'3', '401'
'3', '402'
'3', '403'
'3', '404'
'3', '405'

actions:
# action_id, action_detail, action_required, action_date
'201', 'Fix', 'Open', '2013-01-01 00:00:00'
'202', 'Fix', 'Amend', '2013-01-04 00:00:00'
'203', 'Fix', 'Close', '2013-01-06 00:00:00'
'301', 'Fix', 'Open', '2013-03-01 00:00:00'
'302', 'Fix', 'Review', '2013-04-12 00:00:00'
'401', 'Fix', 'Open', '2013-09-04 00:00:00'
'402', 'Fix', 'Review', '2013-09-12 00:00:00'
'403', 'Fix', 'Close', '2013-09-17 00:00:00'
'404', 'Fix', 'Reopen', '2013-09-21 00:00:00'
'405', 'Fix', 'Close', '2013-09-23 00:00:00'

So far the sql I have is this:

SELECT 
mf.idmy_form, 
mf.summary, 
mf.station, 
a.action_id, 
a.action_required, 
a.action_date 
FROM my_form mf
LEFT JOIN (
SELECT 
   act.action_id, 
   act.action_required, 
   act.action_date, 
   fa.idmy_form 
FROM form_action fa
JOIN actions act 
 ON fa.action_id = act.action_id 
 AND act.action_required = 'Close'
) a 
ON a.idmy_form = mf.idmy_form

This returns the following results:

# idmy_form, summary, station, action_id, action_required, action_date
'1', 'OSR puncture', 'Thornbury', '203', 'Close', '2013-01-06 00:00:00' 
'2', 'Windscreen wiper broke', 'Bristol', NULL, NULL, NULL
'3', 'Out of screenwash', 'Cardiff', '403', 'Close', '2013-09-17 00:00:00'
'3', 'Out of screenwash', 'Cardiff', '405', 'Close', '2013-09-23 00:00:00'

Ideally I would like to only retrieve the most recent by date_completed for each header record where the related actions have a action_required = 'Close'. So eliminating the third row from the results above.

I have tried adding group by on the inner select, but it throws an oracle not a group by expression of missing right parenthesis.

Answer 1

Add a subquery to get the MAX() date. Here are more ways to do this described.

SELECT 
mf.idmy_form, 
mf.summary, 
mf.station, 
a.action_id, 
a.action_required, 
a.action_date 
FROM my_form mf
LEFT JOIN (
SELECT 
   act.action_id, 
   act.action_required, 
   act.action_date, 
   fa.idmy_form 
FROM form_action fa
JOIN actions act 
 ON fa.action_id = act.action_id 
 AND act.action_required = 'Close'
WHERE action_date = (SELECT MAX(action_date) 
                     FROM actions 
                     JOIN form_action USING(action_id)
                     WHERE actions.action_required = 'Close'
                     AND fa.idmy_form = form_action.idmy_form)
) a 
ON a.idmy_form = mf.idmy_form

• see it working live in an sqlfiddle