I need help with my queries of my inventory system

Question

I want tosubtractthe number of entrieswiththe number of outputsof my products,but I have aproblem...the result isnot correct,when
I dothe subtractionto getthe current amount ofmy products.
also I have several inputs and outputs of the same
product.
please I need your help
select e.codpro,(sum(e.cantidad)) as productentry from entradapro e
group by e.codpro
select s.codpro,(sum(s.cantidad)) as productoutput from salidapro s
group by s.codpro
select e.codpro,(sum(e.cantidad)- sum(s.cantidad))as total from entradapro e inner join salidapro s
on e.codpro = s.codpro
group by e.codpro
              

Answer 1

Try this please:
;WITH A1 as
SELECT e.codpro ,
( SUM(e.cantidad) ) AS productentry
FROM entradapro e
GROUP BY e.codpro
), A2 AS
SELECT s.codpro ,
( SUM(s.cantidad) ) AS productoutput
FROM salidapro s
GROUP BY s.codpro
SELECT e.codpro ,
e.productentry - s.productoutput AS total
FROM A1 e
JOIN A2 s ON e.codpro = s.codpro
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