Unpivot in postgres with a column created in the same query - postgresql

Question

I am trying to unpivot a table with PostgreSQL as described here.
My problem is that I am creating a new column in my query which I want to use in my cross join lateral statement (which results in an SQL error because the original table does not have this column).
ORIGINAL QUESTION:
select
"Name",
case
when "Year"='2020' then "Date"
end as "Baseline"
from "test_table"
EDIT: I am using the example from the referred StackOverflow question:
create table customer_turnover
(
customer_id integer,
q1 integer,
q2 integer,
q3 integer,
q4 integer
);
INSERT INTO customer_turnover VALUES
(1, 100, 210, 203, 304);
INSERT INTO customer_turnover VALUES
(2, 150, 118, 422, 257);
INSERT INTO customer_turnover VALUES
(3, 220, 311, 271, 269);
INSERT INTO customer_turnover VALUES
(3, 320, 211, 171, 269);
select * from customer_turnover;
creates the following output
customer_id q1 q2 q3 q4
1 100 210 203 304
2 150 118 422 257
3 220 311 271 269
3 320 211 171 269
(I used the customer_id 3 twice because this column is not unique)
Essentially, what I would like to do is the following: I would like to calculate a new column qsum:
select customer_id, q1, q2, q3, q4,
q1+q2+q3+q4 as qsum
from customer_turnover
and use this additional column in my unpivoting statement to produce the following output:
customer_id turnover quarter
1 100 Q1
1 210 Q2
1 203 Q3
1 304 Q4
1 817 qsum
2 150 Q1
2 118 Q2
2 422 Q3
2 257 Q4
2 947 qsum
3 220 Q1
3 311 Q2
3 271 Q3
3 269 Q4
3 1071 qsum
3 320 Q1
3 211 Q2
3 171 Q3
3 269 Q4
3 971 qsum
As I do not want to have qsum in my final output, I understand that I cannot use it in my select statement, but even if I would use it like this
select customer_id, t.*, q1, q2, q3, q4,
q1+q2+q3+q4 as qsum
from customer_turnover c
cross join lateral (
values
(c.q1, 'Q1'),
(c.q2, 'Q2'),
(c.q3, 'Q3'),
(c.q4, 'Q4'),
(c.qsum, 'Qsum')
) as t(turnover, quarter)
I receive the following SQL error: ERROR: column c.qsum does not exist
How can I produce my desired output?

Answer 1

Not sure to well understand your issue, maybe a subquery can help :
select s.baseline
from
( select
"Name",
case
when "Year"='2020' then "Date"
end as "Baseline"
from "test_table"
) AS s