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SELECT p.product_id, SUM(s.quantity) AS 'sale', SUM(p.quantity) AS 'purchase' FROM purchase p LEFT JOIN sale s ON (s.product_id=p.product_id) GROUP BY p.product_id; This should give you what you need.

You are defining your SQL statement in line 13 however you're not executing it. It would be mysql_query ($sql); Consult your version of php for the exact syntax as some …