我的带有两个JOIN的MySQL查询不起作用

Question

I have written the following query, which does not work. I want to know how to make it work. It is a two-JOIN query which fails to work.

SELECT oc_download.download_id, oc_product_to_download.download_id, oc_download_description.download_id
  FROM oc_download 
         LEFT JOIN oc_product_to_download
                ON oc_download.download_id = oc_product_to_download.download_id 
         LEFT JOIN
                   oc_download.download_id = oc_download_description.download_id
 WHERE oc_product_to_download.product_id = 89

With single JOIN it works, but adding the second JOIN it fails. here is the clean working one-JOIN query:

SELECT oc_download.download_id, oc_product_to_download.download_id, oc_download_description.download_id
  FROM oc_download 
         LEFT JOIN oc_product_to_download
                ON oc_download.download_id = oc_product_to_download.download_id
 WHERE oc_product_to_download.product_id = 89

How should I use multiple JOIN in one single query?

Answer 1

You forgot the table name in the 2nd join

SELECT d.download_id, p.download_id, dd.download_id 
FROM oc_download d
LEFT JOIN oc_product_to_download p ON d.download_id = p.download_id 
LEFT JOIN oc_download_description dd ON d.download_id = dd.download_id 
WHERE p.product_id = 89

And your where clause turns your left join into an inner join . If you don't want that then change your query to

SELECT d.download_id, p.download_id, dd.download_id 
FROM oc_download d
LEFT JOIN oc_product_to_download p ON d.download_id = p.download_id 
                                  AND p.product_id = 89
LEFT JOIN oc_download_description dd ON d.download_id = dd.download_id 

Answer 2

This is your query fixed up a bit, using table aliases and proper join syntax:

SELECT od.download_id, opd.download_id, odd.download_id
FROM oc_download od LEFT JOIN
     oc_product_to_download opd
     ON od.download_id = opd.download_id LEFT JOIN
     oc_download_description odd
     od.download_id = odd.download_id
WHERE opd.product_id = 89;

You are using left join , but this appears to be unnecessary. The on clause is undoing the first outer join, turning it into an inner join (unmatched rows would have a NULL value, which are filtered out by the where clause). In fact, I would guess that your data has well defined foreign key relationships among the columns being joined. If this is the case, you should use inner join (or just join ) for the query:

SELECT od.download_id, opd.download_id, odd.download_id
FROM oc_download od JOIN
     oc_product_to_download opd
     ON od.download_id = opd.download_id JOIN
     oc_download_description odd
     od.download_id = odd.download_id
WHERE opd.product_id = 89;

The left join is misleading because it implies that some keys might not match. You also run the risk of confusing the optimizer.