带WHERE子句的嵌套SQL SUM

Question

I am trying to calculate cost of items sold in a day and total discount given in a day.

Table Structure is

sale Table
invoice_no | bill_date | discount | ...

The child table is (invoice_no is FK with sale table)

sale_items Table
invoice_no | item_no | cost_price | qty

This query works fine

SELECT (SELECT SUM(p.cost_price*p.qty) FROM sale_items p
        WHERE p.invoice_no=s.invoice_no) as tcost_amount
 FROM sale s
 WHERE bill_date= '2017-05-26'

but when I want to calculate discount from parent table, tcost_amount sum up all rows in the child table without bill_date criteria, I am confused.

SELECT SUM(discount), (SELECT SUM(p.cost_price*p.qty)
                       FROM sale_items p
                       WHERE p.invoice_no=s.invoice_no) as tcost_amount
FROM sale s
WHERE bill_date= '2017-05-26'

My question is why bill_date= '2017-05-26' condition is not applying to sale_items in the second case?

Answer 1

At the very least, you need to join the tables. something like this:

select (sum(si.cost_price * si.qty) - s.discount)
from sale s
inner join sale_items si on si.invoice_no = s.invoice_no
where s.bill_date = '2017-05-26'