Chaining endless sql and performance - sql

Question

I am chaining sql according to user filter which is unknown.
For instance he would like to first ask for certain dates :
def filterDates(**kwargs):
q = ('''
SELECT date_num, {subject_col}, {in_col} as {out_col}
FROM {base}
WHERE date_num BETWEEN {date1} AND {date2}
ORDER BY date_num
''').format(subject_col=subject_col,**kwargs)
return q
(base is input query string from previous, see next)
and then he wants to calculate another thing(or many) so we pass the dates filter string query q as base to this query:
WITH BS AS (
SELECT date_num, {subject_col}, {in_col}
FROM {base}
)
SELECT t1.{subject_col},t1.{in_col}, t2.{in_col} - t1.{in_col} as {out_col}
FROM BS t1
JOIN BS t2
ON t1.{subject_col} = t2.{subject_col} AND t2.date_num = {date2}
WHERE t1.date_num = {date1}
''').format(subject_col=subject_col,**kwargs)
Here the {base} is going to be :
base='('+q+')'+'AS base'
Now we can chain queries as much as we want and it works.
How would the engine handle this ? is that means that the efficiency is bad because engine has to make 2 rounds ( instead of having a normal WHERE on the dates? ) how would he optimize this?
Is there a common good practice way to chain unknown number of queries?