SQL: How to refer subexpression from WHERE in SELECT?

Question

Hi, all! I have Postgresql and query

SELECT
  complex_expression()
FROM
  t
WHERE
  complex_expression() != '';

Is there a way to write complex_expression() only once in query and then refer to it? Thanx in advance

Answer 1

select * from
( 
SELECT complex_expression() ce
FROM t
)
where ce != '';

Answer 2

You can subquery

select * from
(
SELECT 
  complex_expression() as exp
FROM 
  t 
)
WHERE 
  exp != '';