Duplicated calculation using subquery

Question

I am trying to reproduce the following calc:

((final_value - init_value) / init_value) * 100

Although I am getting the correct value I would like to ask if there is a better way to do this? At least, to avoid the same subquery twice (init_value)

SELECT (
    (SELECT SUM(x)
     FROM event
     WHERE "year" = 2020) 
     -
    (SELECT SUM(x)
     FROM event
     WHERE "year" = 2000)) 
     /
     (SELECT SUM(x)
      FROM event
      WHERE "year" = 2000)::FLOAT 
     * 100

Answer 1

There may be a an optimal Postgresql-specific method but generally you could use a conditional case expression, something like:

select (Sum(case when "year" = 2020 then x end) - Sum(case when "year" = 2000 then x end))
  / Sum(case when "year" = 2000 then x end)::float * 100
from event
where "year" in (2000,2020);