Question

Given this data, I want to sum (tbl_basket subt), multiply by discount, add shipping charge.

# tbl_basket  order_id  qty    price     subt
                     1    1    10.00    10.00
                     1    4     5.00    20.00
                     1    3    12.00    36.00

# tbl_coupon        id          name discount
                     1     "10% off"    10.00

# tbl_shipping      id                   cost
                     1                   4.99

# tbl_order  coupon_id   shipping_id    total  
                     1             1        ?  # I want to update total

I want to query the basket:

SELECT * FROM tbl_basket WHERE order_id = $order_id

Sum the items, i think like this?

SELECT SUM(subt) FROM tbl_basket WHERE order_id = $order_id

Query the discount

SELECT discount FROM tbl_coupon AS coupon
          INNER JOIN tbl_order AS order
               WHERE coupon.id = order.coupon_id

Q) How do I merge this query into the other one in MySQL?

Q) How do set the value after the discount (new total = total*(100-discount)/100)

I felt it would make the system more robust by summing totals from the database directly, thats why I wanted to do it this way.

Q) Is there a better way to do this?

Thanks for your help.

Answer 1

I agree with evan – doing the calculations in PHP is much better. If nothing else, you can better see what you’re doing!

But if you really want to see what it looks like in SQL, you can try something like this:

SELECT ((select total price query)*(select discount query) + (select shipping query))

Sure, with subqueries you might get everything into a single query statement. The more you add, though, the harder it is to debug. In PHP it’s much easier to assemble the pieces.

Good luck.