Question

I want to do this query:

  SELECT * 
    FROM mail 
   WHERE tag_draft = 0 
     AND (target_id = 2 OR source_id = 2) 
GROUP BY created DESC`

…but it returns no rows. Now, if I do this query:

  SELECT * 
    FROM mail 
   WHERE tag_draft = 0 
     AND target_id = 2 
GROUP BY created DESC

…then it works fine – every row with a target_id of 2 is selected. If I substitute target_id with source_id, it works fine too. The thing is, I want to select rows where the target OR source ID is 2 (2 used as an example here), however running the query first stated

(SELECT * FROM mail WHERE tag_draft=0 AND (target_id=2 OR source_id=2) GROUP BY created DESC)

, with or without inner brackets, returns no rows. I cannot figure out why this will not work, other than I’m doing something wrong with the OR bit.

Some example data:

source_id target_id etc_fields
-------------------------------
2         12        blah
12        2         blah
2         14        blah
2         10        blah
2         2         blah

All the above rows should be displayed in the table. what should NOT be displayed is stuff like:

source_id target_id etc_fields
-------------------------------
10        8         ...
255       16        ...
4         12        ...

Answer 1

Here’s one way, assuming there’s there’s not more than one row with the same MAX(created) for the given condition:

SELECT m.* FROM mail m WHERE m.tag_draft=0 AND (m.target_id=2 OR m.source_id=2) AND
  m.created = (SELECT MAX(created) FROM mail sm 
       WHERE sm.tag_draft=0 AND (sm.target_id=2 OR smsource_id=2))

Or perhaps just

 SELECT m.id,m.target_id,m,m.source_id,MAX(created) FROM mail m 
    WHERE m.tag_draft=0 AND (m.target_id=2 OR m,source_id=2) 
  GROUP BY m.id,m.target_id,m.source_id

Make sure you GROUP BY every column that you select, except the aggregate ( MAX(created) ) , SELECT * won’t do.