SELECT productid, COUNT(productid) AS views FROM products WHERE products.merchantcategory = 'categoryname') ORDER BY views DESC LIMIT 3

SELECT productid, COUNT(productid) AS views FROM products WHERE products.merchantcategory = 'parentcategoryname' ORDER BY views DESC LIMIT 3

IF (query1 returns 0 rows) { execute euery2 and return its results} else { return query1 results }

DELIMITER //
CREATE PROCEDURE query (cat IN VARCHAR, parentcat IN VARCHAR)
BEGIN
  DECLARE count int;

  SELECT COUNT(productid) INTO count
  FROM products
  WHERE merchantcategory = cat;

  IF count > 0 THEN
     SELECT productid, COUNT(productid) AS views
     FROM products
     WHERE merchantcategory = cat
     ORDER BY views DESC LIMIT 3;
  ELSE
     SELECT productid, COUNT(productid) AS views
     FROM products
     WHERE merchantcategory = parentcat
     ORDER BY views DESC LIMIT 3;
  END IF;

END//
DELIMETER ;

mysql_query(db, "call query('categoryname','parentcategoryname')");