How to do an arithmetic operation with aliased column in SQL - mysql

Question

I have a databse table as like below:
id received_by sent_by amount product_id
1 1 2 10 1
2 1 3 12 1
3 2 1 5 1
4 3 1 8 2
Here, received_by and sent_by are two user ID those who are receiving and sending the product respectively. I want to calculate the total amount of each product of a single user by subtracting the sent amount from received amount.
My current query looks like below:
select
product_id,
(received - sent) as quantity,
case(when received_by = 1 then amount end) as received,
case(when sent_by = 1 then amount end) as sent
group by
product_id;
Here I get an error that Unknown column 'received' in 'field list'.
How can I calculate each users inventory/stock?

Answer 1

You can't use the calculated columns in the SELECT list.
Also you need the aggregate function SUM().
One way to do it is with a subquery:
select *, (received - sent) as quantity
from (
select product_id,
sum(case when received_by = 1 then amount else 0 end) as received,
sum(case when sent_by = 1 then amount else 0 end) as sent
from tablename
where 1 in (received_by, sent_by)
group by product_id
) t
Or:
select product_id,
sum(case when received_by = 1 then amount else -amount end) as quantity,
sum(case when received_by = 1 then amount else 0 end) as received,
sum(case when sent_by = 1 then amount else 0 end) as sent
from tablename
where 1 in (received_by, sent_by)
group by product_id
See the demo.

Answer 2

First of all you missed the from clause in your query before group by. Secondly you cannot use column aliases (received, sent) in same select statement.
create table mytable(id int, received_by int, sent_by int, amount int, product_id int);
insert into mytable values(1, 1, 2, 10, 1);
insert into mytable values(2, 1, 3, 12, 1);
insert into mytable values(3, 2, 1, 5, 1);
insert into mytable values(4, 3, 1, 8, 2);
Query:
select product_id, (coalesce(received,0)-coalesce(sent,0)) as Quantity, coalesce(received,0) received,coalesce(sent)sent
from
( select
product_id,
sum(case when received_by = 1 then amount end) as received,
sum(case when sent_by = 1 then amount end) as sent
from mytable
group by
product_id
)t;
Output:
|product_id | Quantity | received | sent|
|-----------|----------|----------|-----|
| 1 | 17 | 22 | 5|
| 2 | -8 | 0 | 8|
db<>fiddle here