Postgres query : using previous dynamically created colum value in next - postgresql

Question

I'm trying to implement what I have in code as a postgres query.
The following example isn't exactly what we're trying to do but I hope it shows how I'm trying to use the value from a previously calculated row in the next.
A sample table to help me demonstrate what I'm trying to do :
test=# select * from test ;
id | field1 | field2 | field3 | score
----+--------+--------+--------+-------
1 | 1 | 3 | 2 | 1.25
2 | 1 | -1 | 1 |
3 | 2 | 1 | 5 |
4 | 3 | -2 | 4 |
Here's the query in progress:
select id,
coalesce (
score,
case when lag_field3 = 2 then 0.25*(3*field1+field2) end
) as new_score
from (
select id, field1, field2, field3, score,
lag (field3) over (order by id) as lag_field3
from test
) inner1 ;
Which returns what I want so far ...
id | new_score
----+-----------
1 | 1.25
2 | 0.5
3 |
4 |
The next iteration of the query:
select id,
coalesce (
score,
case when lag_field3 = 2 then 0.25*(3*field1+field2) end,
case when field1 = 2 then 0.75 * lag (new_score) end
) as new_score
from (
select id, field1, field2, field3, score,
lag (field3) over (order by id) as lag_field3
from test
) inner1 ;
The difference is this :
case when field1 = 2 then 0.75 * lag (new_score) end
I know and understand why this won't work.
I've aliased the calculated field as new_score and when field1 = 2, I want 0.75 * the previous rows new_score value.
I understand that new_score is an alias and can't be used.
Is there some way I can accomplish this? I could try to copy that expression, wrap a lag around it, alias that as something else and try to work with that but that would get very messy.
Any ideas?
Many thanks.

Answer 1

Postgres lets you use windows in CASE statements. Probably you were missing the OVER (ORDER BY id) part. You can also define different windows but you can't use windows in conjunction with GROUP BY. Also, it won't let you use annidate windows, so you have to write down some subqueries or CTEs.
Here's the query:
SELECT id, COALESCE(tmp_score,
CASE
WHEN field1 = 2
THEN 0.75 * LAG(tmp_score) OVER (ORDER BY id)
-- missing ELSE statement here
END
) AS new_score
FROM (
SELECT id, field1,
COALESCE (
score,
CASE
WHEN LAG(field3) OVER (ORDER BY id) = 2
THEN 0.25*(3*field1+field2)
END
) AS tmp_score
FROM test
) inner1
The code to create and populate the table:
CREATE TABLE test(
id int,
field1 int,
field2 int,
field3 int,
score numeric
);
INSERT INTO test VALUES
(1, 1, 3, 2, 1.25),
(2, 1, -1, 1, NULL),
(3, 2, 1, 5, NULL),
(4, 3, -2, 4, NULL);
The query returns this output:
id | new_score
----+-----------
1 | 1.25
2 | 0.50
3 | 0.3750
4 |