Using result of a SQL case to compare and return another case - sql

Question

I'm trying to compare the result of a sum which returns an amount, by using a case that would return the result of this comparison:
SELECT u.name, u.account,
sum (case u.account
when t.sent_to then t.quantity * -1
else t.quantity end) as quantity_sent,
case when quantity_sent > u.available then 'YES'
else 'NO' end as analysis_result
from users as u inner join trans as t
on u.account in (t.sent_to, t.received_by)
group by u.name, u.account;
Whenever I call quantity_sent back, I get an "Invalid column name quantity_sent". Is there any other way I can use the result on this newly created column to compare inside a case?

Answer 1

Try this-
SELECT
u.name, u.account,
SUM (
CASE u.account
WHEN t.sent_to THEN t.quantity * -1
ELSE t.quantity
END
) AS quantity_sent,
CASE
WHEN SUM (
CASE u.account
WHEN t.sent_to then t.quantity * -1
ELSE t.quantity
END ) > u.available THEN 'YES'
ELSE 'NO'
END as analysis_result
FROM users as u
INNER JOIN trans as t
ON u.account in (t.sent_to, t.received_by)
GROUP BY u.name, u.account;

Answer 2

You need to repeat the expression or use a subquery:
select name, account, quantity_sent,
(case when quantity_sent > u.available then 'YES'
else 'NO'
end) as analysis_result
from (select u.name, u.account,
sum(case u.account
when t.sent_to then t.quantity * -1
else t.quantity
end) as quantity_sent
from users u inner join
trans t
on u.account in (t.sent_to, t.received_by)
group by u.name, u.account
) u;
This is part of the general rule in SQL that you cannot re-use a column alias in the SELECT where it is defined. There is a simple reason for this: SQL does not guarantee the order of evaluation of the expressions in the SELECT.