Sum values from 2 tables and subtract it to each other - sql-server

Question

I have 2 tables, ord_tbl and pay_tbl. o_tbl with these data.
ord_tbl
invoice | emp_id | prod_id | amount
123 | 101 | 1 | 1000
123 | 101 | 2 | 500
123 | 101 | 3 | 500
124 | 101 | 2 | 300
125 | 102 | 3 | 200
pay_tbl
invoice | new_invoice | amount
123 | 321 | 300
123 | 322 | 200
124 | 323 | 300
125 | 324 | 100
I would like the selection statement to give me this result
invoice | emp_id | orig_amt | balance | status
123 | 101 | 2000 | 1500 | unsettled
The invoice that has 0 balance will not be included anymore. I know that I have to use the join and sub queries here but I don't even know how to start it! For me, as a beginner, this is very complex already. This is what I tried so far...
SELECT
ord_tbl.invoice,
SUM(ord_tbl.amount) As 'origAmt',
SUM(pay_tbl.amount) As 'payAmt',
origAmt - payAmt As 'bal'
FROM
ord_tbl
INNER JOIN pay_tbl
ON ord_tbl.invoice = pay_tbl.invoice
WHERE
ord_tbl.emp_id = #emp_id AND
bal != 0
GROUP BY
ord_tbl.invoice

Answer 1

You need to prepare your data joining on a non unique field will lead to a cross JOIN,that`s why you get 4000
;WITH CTE as
(SELECT ot.invoice,MAX(ot.emp_id) as emp_id,SUM(ot.amount) as origAmt FROM ord_tbl ot GROUP BY ot.invoice),
CTE2 as
( SELECT pt.invoice,SUM(pt.ammount) as payAmt FROM pay_tbl pt GROUP BY pt.invoice)
SELECT CTE.invoice,CTE.emp_id,CTE.origAmt,CTE.origAmt-NULLIF(CTE2.payAmt,0) as bal,'unsettled' as status
FROM
CTE LEFT JOIN CTE2 ON CTE.invoice=CTE2.invoice
AND CTE.emp_id=101 AND CTE.origAmt-NULLIF(CTE2.payAmt,0)>0