Дмитрий STEM

SELECT *,
sum(
	case when ( 	  ( products_vendors.quantity > 0 ) 
			        and (    ( product_vendors.vendor_id <> 'X' )
					          or (   ( products_vendors.vendor_id = 'X' ) 
						           and ( products.type = 'X' )
					             )
				          )
			  ) then 1 else 0 end
) as sum1,
FROM products 
LEFT JOIN products_vendors 
ON products.id=products_vendors.product_id 
WHERE product_vendors.vendor_id IN (‘a’, ‘b’, ‘x’) 
GROUP BY products.id 
HAVING ( sum1 > 0 );