Feature Request: Adding Support for CTEs

[frodo821]

Hi, folks. I want support for CTEs (common table expressions) as a new feature of PonyORM.

Motivation

Some queries require using CTEs. For instance, a query using window functions as a filter condition needs CTE like this:

WITH q(`id`, `sum_price`) AS (
  SELECT `id`, sum(`price`) OVER (order by `last_updated` desc) AS `count` FROM `items`
)
SELECT `id` FROM `q`
WHERE `sum_price` < 20;

As a limitation over window functions of standard SQL, it never allows using window functions in WHERE/HAVING clause. Because of this reason, CTE is inevitable for this query.