Limit a Left Join on the first table

问题:

I have two tables: gems and gemdetail that are left joined. I am trying to limit the LEFT JOIN to 10 records in the gems table. There are 2 other tables joined (gemreply and users) as well, but they do not contribute to the problem. The following does not work:

SELECT gems.gemid, gems.title, r.tot, gemdetail.filename FROM ((gems 
LEFT JOIN (SELECT gemid, COUNT(*) AS tot FROM gemreply GROUP BY gemid) AS r ON gems.gemid = r.gemid) 
LEFT JOIN gemdetail ON gems.gemid = gemdetail.gemid) 
LEFT JOIN users ON gems.userid = users.userid 
WHERE gems.grade = '7' ORDER BY gems.gemid LIMIT 0, 10;

This limits the total number of returned rows to 10, but since there are multiple detail records for each gem, I am left with fewer than 10 gem records. I've read every "LIMIT" post, but haven't found this situation.

UPDATE 1: OK - thanks to jviladrich - it worked. Here's the code:

SELECT gems.gemid, gems.title, r.tot, gemdetail.filename  
FROM ((gems 
INNER JOIN (SELECT gems.gemid from gems WHERE gems.grade = '7'  ORDER BY gems.gemid LIMIT 0, 10) g
ON (gems.gemid = g.gemid)
LEFT JOIN (SELECT gemid, COUNT(*) AS tot FROM gemreply GROUP BY gemid) AS r ON gems.gemid = r.gemid) 
LEFT JOIN gemdetail ON gems.gemid = gemdetail.gemid) 
LEFT JOIN users ON gems.userid = users.userid ;

UPDATE 2: The code from yogeshr below also works - probably the one I'll go with. Thanks to both of you!

回答1:

Something like that

   SELECT * FROM A
      INNER JOIN ( SELECT * FROM A WHERE A.FIELD1='X' ORDER BY A.FIELD2 LIMIT 10) X
             ON (A.KEYFIELD=X.KEYFIELD)
      LEFT JOIN B ON (A.FIELD = B.FIELD)
      LEFT JOIN C ON (A.FIELD = C.FIELD)

回答2:

Try this:

SELECT g.gemid, g.title, r.tot, gemdetail.filename
FROM (SELECT * FROM gems WHERE grade = '7' LIMIT 10) g
LEFT JOIN (SELECT gemid, COUNT(*) AS tot FROM gemreply GROUP BY gemid) r
          ON r.gemid = g.gemid
LEFT JOIN gemdetail ON g.gemid = gemdetail.gemid
LEFT JOIN users ON g.userid = users.userid
ORDER BY g.gemid;

This should work.