Postgres select max id only if value of another column is negative - sql

Question

So i need to write a query that will return some values but only if the latest record in that table for any user is less than 0. Here is what if been playing around with:
select
et.id,
et.user_id,
et.amount,
et.trans_type,
COALESCE(et."endingBalance", 0) AS current_balance,
et."processId",
upu.email,
et.created_at
from "users-permissions_user" upu
join employer_transactions et
on upu.id = et.user_id
and et.id = (
select max(et2.id) from employer_transactions et2 where et2."endingBalance" < 0
)
and this is what is is returning:
id
user_id
amount
trans_type
current_balance
email
created_at
1946
333
150
CREDIT
-900.31
...
...
but if i run this query to test that query for that user_id:
select
id,
user_id,
amount ,
trans_type,
"endingBalance"
from employer_transactions et
where user_id = 333
order by id desc;
here is what i see:
id
user_id
amount
trans_type
ending_balance
1952
333
3
DEBIT
1297.31
1951
333
1
DEBIT
1299.31
1950
333
2
DEBIT
1298.31
1947
333
400
CREDIT
1300.31
1946
333
150
CREDIT
-900.31
so in this case what im looking for was for this query to have returned nothing because the record with the highest id is not negative
but lets say the sample data set is this:
id
user_id
amount
trans_type
ending_balance
900
333
3
DEBIT
-1297.31
899
333
1
DEBIT
1299.31
700
222
2
DEBIT
-1298.31
699
222
400
CREDIT
1300.31
600
111
150
CREDIT
900.31
599
111
150
CREDIT
-800.31
then what im looking for my query to return is
id
user_id
amount
trans_type
current_balance
email
created_at
900
333
3
DEBIT
-1297.31
...
...
700
222
2
DEBIT
-1298.31
...
...
because those were the latest records for that particular user_id and the current_balance was negative but noting for user_id: 111 becasue while yes there was a negative record but it wasnt the latest record for that user_id

Answer 1

SELECT T.*
FROM
(
SELECT *, RANK() OVER(PARTITION BY user_id ORDER BY id DESC) RNK
FROM trans_tbl
) T
WHERE T.RNK = 1 AND T.ending_balance < 0;
Fiddle

Answer 2

A common technique, for me, is to do a subquery that finds a desired subset. After that, do more filtering.
with ids as (
select max(id) as last_id
from t
group by user_id
)
select *
from t
where end_balance < 0
and id in (select last_id from ids)
;
The above gives desired output, given that the `id`'s are unique and form an ascending sequence.