MySQL Inner Join involving three tables - php

Question

I am working with four tables:
query,
store,
cluster_group,
tv_region
I want to retrieve query records belonging to a particular TV Region record.
A query record either belongs to a record in the store or cluster_group tables. The query table has 'store_id' and 'cluster_group_id' columns. Either will be null whilst the other will refer to a record in the store or cluster_group table. The store and cluster_group tables both have a 'tv_region_id' column.
To retrieve query records that belong to a TV Region record that has an id = 2, I wrote the following SQL statement:
SELECT query.id AS query_id, cluster_group.name as cluster_name, cluster_group.tv_region_id as cluster_tv_region, store.store_name
FROM query
INNER JOIN cluster_group
ON cluster_group.id=query.cluster_group_id
INNER JOIN store
ON store.id=query.store_id
WHERE cluster_group.tv_region_id = 2
AND store.tv_region_id = 2;
The problem is that it returns zero records even though there are query records that belong to the specified TV Region (via a cluster group or store record). I may have misunderstood how 'inner join' works.
I'm working with Doctrine 2 and have attempted the query using the left join but it still returns nothing, I guess this is because its returning null values too. How do I get it to return only the query records I am interested in and no null values?
Appreciate if someone can point me in the right direction to retrieve the relevant query records.

Answer 1

You will need to use OUTER joins, or possibly LEFT
INNER joins will only return data where there is a matching record on both sides. As you state in your question you only get a match on one side or the other, therefore there is never going to be a matching record across both store and cluster
Jeff Atwood has a good explanation of the join types on his blog http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

Answer 2

Try this:-
SELECT query.id AS query_id, cluster_group.name as cluster_name, cluster_group.tv_region_id as cluster_tv_region, store.store_name
FROM query
LEFT OUTER JOIN cluster_group
ON cluster_group.id=query.cluster_group_id
AND cluster_group.tv_region_id = 2
LEFT OUTER JOIN store
ON store.id=query.store_id
AND store.tv_region_id = 2
EDIT - no experience of doctrine, but you could try using a UNION:-
SELECT query.id AS query_id, cluster_group.name as cluster_name, cluster_group.tv_region_id as cluster_tv_region, NULL AS store_name
FROM query
INNER JOIN cluster_group
ON cluster_group.id=query.cluster_group_id
AND cluster_group.tv_region_id = 2
UNION
SELECT query.id AS query_id, NULL as cluster_name, NULL as cluster_tv_region, store.store_name
FROM query
INNER JOIN store
ON store.id=query.store_id
AND store.tv_region_id = 2

Answer 3

It works like this for me:
from table1 inner join table2 on table1.id = table2.id
inner join table3 on table1.id = table3.id