If I use an alias in a SELECT clause, how do I refer back to that alias?

Question

I want to do something like this:

SELECT round(100*(col_a - col_b)/col_a, 1) as Foo, Foo - col_c as Bar
FROM my_table
WHERE...;

However, I get an error saying Foo is unknown. Since Foo is derived from some calculations on a bunch of other columns, I don't want to repeat the formula again for Bar. Any work-arounds?

Answer 1

SELECT Foo, Foo - col_c as Bar
from (
SELECT round(100*(col_a - col_b)/col_a, 1) as Foo,  col_c
FROM my_table
WHERE...

) t;