Join table with MAX value from another

Question

select * from order
-------------------
|orderID|productID|
-------------------
|  1    |  234    |
|  2    |  234    |
|  3    |  123    |
-------------------

select *         


        

Answer 1

when you need to use aggregate you must use group by and in compare condtion use Having

 SELECT orderID, productID, MAX(cost_price)
 FROM order LEFT JOIN product_supplier 
 ON order.productID=product_supplier.productID having cost_price=MAX(cost_price) group by productID ;

Answer 2

The canonical way of approaching this is to use a subquery to identify the products and their maximum prices from the product_supplier table, and then to join this subquery to order to get the result set you want.

SELECT t1.orderID,
       t1.productID,
       COALESCE(t2.cost_price, 0.0) AS cost_price  -- missing products will appear
FROM order t1                                      -- with a zero price
LEFT JOIN
(
    SELECT productID, MAX(cost_price) AS cost_price
    FROM product_supplier
    GROUP BY productID
) t2
    ON t1.productID  = t2.productID AND
       t1.cost_price = t2.cost_price