具有多个表的联结表上的MySQL SELECT查询

Question

I have four tables, assuming they have only an id as a column each.

• listings
• feature
• location
• l_f_location as a junction table with FKs on listingId , featureId , locationId

I try to query the l_f_location table for a given set of locationIds (logical AND match) and want as a result only those listings which match ALL the locationIds , eg (7 AND 9 AND 10).

l_f_location looks like

listingId featureId locationId
10        5         7
10        7         7
10        8         9
11        4         7
11        8         9
11        9         10
11        12        14

The goal is to retrieve only listingId 11 in this case - matching the set of locationIds 7, 9 and 10.

I tried the following query

" SELECT id, COUNT(*) as num FROM l_f_locations WHERE locationId IN ( 7, 9, 10) GROUP by listingId, locationId HAVING num = 3 "

But that gives wrong values of count as the grouping kicks in.

A similar query works perfectly on a simpler junction table eg only l_location with eg " SELECT id, COUNT(*) as num FROM l_location WHERE locationId IN ( 7, 9, 10) GROUP by listingId HAVING num = 3 ".

Rgds, P.

Answer 1

I'm assuming that the same listingId and locationId can have multiple featureId s? Otherwise it's quite a bit easier.

If that is the case:

SELECT listingId, COUNT(*) as num 
FROM
(SELECT DISTINCT listingId, locationId
 FROM l_f_locations
 WHERE locationId IN ( 7, 9, 10)) AS sub
GROUP by listingId, HAVING num = 3