PSQL average with IF/ELSE - if-statement

Question

I have a psql table with columns including: year, value, ... and so on.
I want to do something like this:
select
CASE WHEN avg(value) >=0 then avg(value)
ELSE -999
END
from my_table
where year >= 2000 and year < 2005 and value >= 0
So I want my average to ignore any years that have negative value, but for cases where all years have negative value, I want to return -999.
This query runs but doesn't return -999 in the case where all values are negative.

Answer 1

If I've understood you correctly you're looking for something like:
select
coalesce(avg(value) filter (when value >= 0), -999)
from my_table
where year >= 2000 and year < 2005;
which returns -999 if all values are negative or there are zero rows that match the WHERE clause. avg returns null if it doesn't get any input rows.
The filter syntax works only on newer PostgreSQL versions. For older ones you must use avg(case when value >= 0 then value end).

Answer 2

you have given in the where clause value>=0 This will only compute the average of non negative values, hence you will never get the average of negative values
where year >= 2000 and year < 2005 and value >= 0
Guess what you are trying to do is this
select
CASE WHEN value >=0 then
(select avg(value) from t_table where value>=0)
ELSE -999
END
from table
where year >= 2000 and year < 2005;