Question

Here is the db schema

Tables-

I want to write a query to search all files which have a metadata value like(‘abc’)
and get a specific corresponding metadata value for that file.

So for above search for all files which have abc1 as one of the values and give me its corresponding ‘geo’ key’s value.
I know it can be done by a subquery. But want to find the most efficient way.

Answer 1

Could look like this. Should be the most efficient way, too.

SELECT m0.m_value
FROM   metadata m
JOIN   file_metadata fm ON fm.m_foreign_key = m.m_id
JOIN   file_metadata fm0 ON fm0.f_foreign_key = fm.f_foreign_key
--                      AND fm0.m_foreign_key <> fm.m_foreign_key
--  may or may not be necessary, depending on the selectivity of 'geo'
JOIN   metadata m0 ON m0.m_id = fm0.m_foreign_key
WHERE  m.m_value = 'abc1'
AND    m0.key = 'geo'

Be sure to assign aliases to multiple instances of the same table in one query.

For an overview of related query techniques see this lineup: How to filter SQL results in a has-many-through relation.