MySQL using COUNT, LEFT JOIN and GROUP by returning undesired results

Question

I need some help to get the desired results, which would in this case, be 7 (the number of rows in the products table that would match).

What I am instead getting is 7 rows with a count based on the the number of rows returned in the LEFT JOIN.

SELECT count(p.id) as theCount 
            FROM products p 
            left join tablea a on p.id = a.productId 
            left join tableb b on p.id = b.productId 
            WHERE (a.col = 'val' or b.col = 'val')
group by p.id

If I do not group by p.id, I get back 28 rows, which is all of the rows from the LEFT JOIN.

I know it's something simple, but I can't figure it out.

Thanks.

Answer 1

select count(distinct p.id), perhaps? Since you're pulling from two different tables, you're going to get a mismash of (p.id, a.col, b.col) being (xxx, null, yyy) and (xxx, yyy, null)

Answer 2

You shouldn't join the one-to-many relationships if all you want is the count of products.

Put your filter condition in the WHERE clause.

SELECT count(*) as theCount 
        FROM products p 
        WHERE p.id IN (
             SELECT a.productId
             FROM tablea a
             WHERE a.productId = p.id AND a.col = 'val'

             UNION

             SELECT b.productId
             FROM tableb b
             WHERE b.productId = p.id AND b.col = 'val'
        )