Question

I have some tables that have ‘has and belongs to many’ type relationships using join tables. The main tables are countries, programs and listings. The basic structure is:

countries
  -id
  -name

programs
  -id
  -name

listings
  -id
  -title

countries_programs
  -country_id
  -program_id

listings_programs
  -listing_id
  -program_id

listings_countries
  -listing_id
  -country_id

I have been doing the following to list the programs for a country when given the country id value (in this case 5):

SELECT programs.* 
FROM programs 
LEFT JOIN countries_programs ON programs_id = countries_programs.program_id 
LEFT JOIN countries ON countries.id = countries_programs.country_id 
WHERE countries_id = '5'

What I need to do is only return programs for the country only if the programs for the specific country actually have any listings that are also in that country. So it should return only the case where the programs are in the country specified and have listings that are in that program and also in that country.

If a program for the specified country has no listings, then I don’t want it returned. I’ve been trying various combinations, but can’t seem to get this to work. Does anyone see how this could be done?

I think I need to join the listings table, but nothing I’ve tried has come close.

Answer 1

To avoid returning duplicate data, I’d use an EXISTS clause. Also, switch to INNER joins to satisfy the country requirement.

SELECT p.* 
FROM programs p
INNER JOIN countries_programs cp
    ON p.id = cp.program_id 
WHERE cp.country_id = 5
AND EXISTS (
    SELECT 1 FROM listings_countries lc
    INNER JOIN listings_programs lp
        ON lc.listing_id = lp.listing_id
    WHERE lc.country_id = c.id
    AND lp.program_id = p.id
)

I’ve omitted the join to countries as you’re only dealing with the country ID which is available in countries_programs.