Question

mysql> select * from orders;
+---------+------------+------------+------------+----------+----------+
| orderid | customerid | orderdate  | shipdate   | shipping | salestax |
+---------+------------+------------+------------+----------+----------+
|       1 |          1 | 2008-03-31 | 2008-03-31 |     4.99 |     5.00 | 
|       2 |          1 | 2008-04-01 | 2008-04-02 |     5.99 |     5.00 | 
|       3 |          2 | 2008-04-01 | 2008-04-02 |     3.99 |     6.00 | 
|       4 |          3 | 2008-04-02 | 2008-04-02 |     6.99 |     7.50 | 
+---------+------------+------------+------------+----------+----------+
4 rows in set (0.02 sec)
mysql> select * from orderinfo;
+---------+------------+-----+-------+----------+
| orderid | isbn       | qty | price | detailid |
+---------+------------+-----+-------+----------+
|       1 | 0929306279 |   1 | 29.95 |        1 | 
|       1 | 0929306260 |   1 | 49.95 |        2 | 
|       2 | 0439357624 |   3 | 16.95 |        3 | 
|       3 | 0670031844 |   1 | 34.95 |        4 | 
|       4 | 0929306279 |   1 | 29.95 |        5 | 
|       4 | 0929306260 |   1 | 49.95 |        6 | 
|       4 | 0439357624 |   1 | 16.95 |        7 | 
|       4 | 0670031844 |   1 | 34.95 |        8 | 
+---------+------------+-----+-------+----------+
8 rows in set (0.00 sec)

im trying to multiply the qty and price from the orderinfo table then add the shipping from the order table when i try to multiple them it works fine but when I add the shipping I get an incorrect value

this works

mysql> select orders.orderid,sum(orderinfo.qty*orderinfo.price) as order_total from orders
    -> inner join orderinfo
    -> on orders.orderid=orderinfo.orderid
    -> group by orderid;
+---------+-------------+
| orderid | order_total |
+---------+-------------+
|       1 |       79.90 | 
|       2 |       50.85 | 
|       3 |       34.95 | 
|       4 |      131.80 | 
+---------+-------------+
4 rows in set (0.00 sec)

this gives an incorrect value

mysql> select orders.orderid,sum(orderinfo.qty*orderinfo.price+orders.shipping) as order_total from orders
    -> inner join orderinfo
    -> on orders.orderid=orderinfo.orderid
    -> group by orderid;
+---------+-------------+
| orderid | order_total |
+---------+-------------+
|       1 |       89.88 | 
|       2 |       56.84 | 
|       3 |       38.94 | 
|       4 |      159.76 | 
+---------+-------------+
4 rows in set (0.00 sec)

Answer 1

You are applying the shipping rate inside the SUM() aggregate, when really you need to add it afterward, to the result of the SUM(). This has the effect of adding the shipping value for each row of the aggregated group, that is, double shipping for orderid = 1 and 4x shipping for orderid = 4.

You can do this by adding shipping to the GROUP BY (actually MySQL doesn’t require this, but it is good practice).

SELECT
  orders.orderid,
  /* Add `shipping` outside the SUM() aggregate */
  SUM(orderinfo.qty*orderinfo.price) + orders.shipping AS order_total 
FROM orders
  INNER JOIN orderinfo ON orders.orderid = orderinfo.orderid
GROUP BY
  orderid,
  shipping

Here’s an example on SQLFiddle.com.