PostgreSQL Join – How to Merge Left Outer Join Column

Basically, I think you should just get the max date, if that is all you are looking for here, using your same filter, etc. You already have STATUS (=COMP) and WONUM (JOIN). If you needed the whole record from this table, and it was more complicated than this, I would recommend the oracle inline analytic functions with over/partition by logic to filter by the max date.

SELECT *
FROM   WORKORDER
       LEFT OUTER JOIN (SELECT WONUM AS STATUSWONUM 
                               , STATUS AS STATUS
                               , MAX(CHANGEDATE) AS STATUSCHANGEDATE
                        FROM   WOSTATUSHISTORY
                        WHERE  STATUS = 'COMP'
                        GROUP  BY WONUM, STATUS)LASTCOMPLETE
         ON ( WORKORDER.WONUM = LASTCOMPLETE.STATUSWONUM ) 

;

First consider a query that computes which rows are actually relevant from tablethree. With the assumption that with "most recently entered result" you mean "most recent enddate" the following query would gather the appropriate rows:

SELECT sid, MAX(enddate) FROM `tablethree` GROUP BY sid

Now you can build a join to retrieve not only sid, but all of the data of tablethree:

SELECT a.*
FROM tablethree a
INNER JOIN (
  SELECT sid, MAX(enddate) FROM `tablethree` GROUP BY sid
) b
ON a.sid = b.sid AND a.enddate = b.enddate

This is the result set you actually want to "left join in". You have to insert this into your original query:

SELECT t1.*
FROM tableone AS t1
INNER JOIN tabletwo AS t2
  ON t1.cid = t2.id
LEFT OUTER JOIN (
  SELECT a.*
  FROM tablethree a
  INNER JOIN (
    SELECT sid, MAX(enddate) FROM `tablethree` GROUP BY sid
  ) b
  ON a.sid = b.sid AND a.enddate = b.enddate
) AS t3
  ON t3.sid = t2.sid
WHERE t1.fieldone = 1 
  AND t1.odate NOT BETWEEN t3.startdate AND t3.enddate

What should also work is the following:

SELECT t1.*
FROM tableone AS t1
INNER JOIN tabletwo AS t2
  ON t1.cid = t2.id
LEFT OUTER JOIN tablethree AS t3
  ON t3.sid = t2.sid
LEFT OUTER JOIN (
  SELECT sid, MAX(enddate) FROM `tablethree` GROUP BY sid
) mostrecent
  ON t3.sid = mostrecent.sid AND t3.enddate = mostrecent.enddate

WHERE t1.fieldone = 1 
  AND t1.odate NOT BETWEEN t3.startdate AND t3.enddate
  AND mostrecent.enddate IS NULL

This includes both tablethree and the new SELECT as left joins, and sorts out the rows where mostrecent.enddate IS NULL (meaning those rows which are actually not most recent). This should lead to the same result, but MySQL may be able to compute this result a little faster. EXPLAIN on both queries should reveal possible differences in computation.