Whats wrong with this nested query? - sql

Question

I am trying to write a query to return the id of the latest version of a market index stored in a database.
SELECT miv.market_index_id market_index_id from ref_market_index_version miv
INNER JOIN ref_market_index mi ON miv.market_index_id = mi.id
WHERE mi.short_name='dow30'
AND miv.version_num = (SELECT MAX(m1.version_num) FROM ref_market_index_version m1 INNER JOIN ref_market_index m2 ON m1.market_index_id = m2.id )
The above SQL statement can be (roughly) translated into the form:
SELECT some columns FROM SOME CRITERIA MATCHED TABLES
WHERE mi.short_name='some name'
AND miv.version_num = SOME NUMBER
What I don't understand is that when I supply an actual number (instead of a sub query), the SQL statement works - also, when I test the SUB query used to determine the latest version number, that also works - however, when I attempt to use the result returned by sub query in the outer (parent?) query, it returns 0 rows - what am I doing wrong here?
Incidentally, I also tried an IN CLAUSE instead of the strict equality match i.e.
... AND miv.version_num IN (SUB QUERY)
That also resulted in 0 rows, although as before, when running the parent query with a hard coded version number, I get 1 row returned (as expected).
BTW I am using postgeresql, but I prefer the solution to be db agnostic.

Answer 1

The problem is probably that the max(version_num) doesn't exist for 'dow30'.
Try the following correlated subquery:
SELECT miv.market_index_id market_index_id
from ref_market_index_version miv INNER JOIN
ref_market_index mi
ON miv.market_index_id = mi.id
WHERE mi.short_name='dow30' AND
miv.version_num = (SELECT MAX(m1.version_num)
FROM ref_market_index_version m1 INNER JOIN
ref_market_index m2
ON m1.market_index_id = m2.id
where m1.short_name = 'dow30'
)
I added the where clause in the subquery.