Postgresql reference a variable within statement

I come across this scenario frequently in sql and I have seen it done, but would like to summarize the case when/when not to reference a new variable within a statement.

1. This doesn’t work –

SELECT 
COUNT(id) AS total_id,
COUNT(DISTINCT id) AS distinct_id,
total_id - distinct_id AS var
FROM mytable

2. but this does –

SELECT 
COUNT(id) AS total_id,
COUNT(DISTINCT id) AS distinct_id,
COUNT(id) - COUNT(DISTINCT id) AS var
FROM mytable

How can I implement scenario 1?

Answer

You can use a subquery.

PostgreSQL:
SELECT total_id, distinct_id, (total_id + distinct_id) as var FROM
(
    SELECT COUNT(id) AS total_id, COUNT(DISTINCT id) AS distinct_id
    FROM mytable
) as m