Self-join of a subquery

Question

I was wondering, is it possible to join the result of a query with itself, using PostgreSQL?

Answer 1

You can do so with WITH:

WITH subquery AS(
    SELECT * FROM TheTable
) 
SELECT *
FROM subquery q1
JOIN subquery q2 on ...

Or by creating a VIEW that contains the query, and joining on that:

SELECT *
FROM TheView v1
JOIN TheView v2 on ...

Or the brute force approach: type the subquery twice:

SELECT *
FROM (
    SELECT * FROM TheTable
) sub1
LEFT JOIN (
    SELECT * FROM TheTable
) sub2 ON ...