My MySQL query with two JOINs does not work - mysql

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.