I need to use the computed values (after 'as' keyword) of two functions in sql in the next statement. I want to use it because the time of computations decreases x2 in this case.
I have the following statement:
SELECT f1() as f_1, f2() as f_2, f_1 - f_2 as f1_minus_f2 FROM mytable
where f1(), f2() - some functions
1) You can use expression itself (if functions are deterministic):
SELECT f1() as f_1, f2() as f_2, f1() - f2() as f1_minus_f2
FROM mytable
2) Use subquery:
SELECT sub.f_1, sub.f_2, sub.f_1 - sub.f_2 as f1_minus_f2
FROM (
SELECT f1() as f_1, f2() as f_2
FROM mytable
) sub
The reason you cannot use it as you want is all-at-once rule:
"All-at-Once Operations" means that all expressions in the same logical query process phase are evaluated logically at the same time.
