MySQL-SELECT与LEFT JOIN优化

Question

I have 3 tables to select from. 2 of them are always necessary ( tbl_notes , tbl_clients ) while the 3rd is optional ( tbl_notes_categories ).

I've always used a LEFT JOIN in my queries with questionable correlating records to the primary table.

But I'm not getting any results with the query below.

Would someone point out how I'm using the LEFT JOIN incorrectly?

SELECT n.*, c.clientname, nc.notecategoryname 
FROM tbl_notes n, tbl_clients c 
LEFT JOIN tbl_notes_categories nc ON n.categoryid = nc.categoryid 
WHERE n.clientid = c.clientid 
AND c.clientid = 12345 
ORDER BY n.dateinserted DESC

In fact, I'm getting a sql error. #1054 - Unknown column 'n.categoryid' in 'on clause'

categoryid certainly does exist in tbl_notes

I probably need to brush up on how JOINS really work. I'm guessing I cannot have a LEFT JOIN with 2 database tables before it?

On a side note, I can foresee times when there will be multiple required tables, with several optional tables. (in this case tbl_notes_categories is optional)

Answer 1

Assuming the column categoryid exists in the tbl_notes table...

Try rewriting the query to use the JOIN syntax, rather than using the old-school comma as the join operator. (If the problem isn't a misnamed column, I suspect the problem is in mixing the two types of syntax... but this is just a suspicion, I have no reason to test mixing old-style comma joins with JOIN keywords.)

I'd write the statement like this:

SELECT n.*, c.clientname, nc.notecategoryname 
  FROM tbl_notes n
  JOIN tbl_clients c 
    ON n.clientid = c.clientid 
  LEFT
  JOIN tbl_notes_categories nc 
    ON nc.categoryid = n.categoryid
 WHERE c.clientid = 12345
 ORDER BY n.dateinserted DESC

(Actually, I would specify the individual columns to return from n , rather than using n.* , but that's just a style preference, not a SQL syntax requirement.)