MySQL-query, 3 tabellen, t1-t2 en update vervolgens t3. Hoe kan ik dit het meest efficiënt doen?

Beantwoord uw opmerking hieronder:

UPDATE inventory a 
JOIN (select ProdName, sum(Quantity) Quantity from inprod group by ProdName) b on a.ProdName=b.ProdName 
JOIN (select ProdName, sum(Quantity) Quantity from outprod group by ProdName) c on a.ProdName=c.ProdName      
SET a.Quantity = c.Quantity-b.Quantity
WHERE a.ProdName='Chocolate'