(Enhancement) Use a sql query as an expression

[niviksha]

Is there a way for ibis to have an expression simply be a static, pre-existing SQL query? e.g. today, the primary workflow for ibis (in our omnisci/mapd backend) is to generate sql via the compilation path, and i get that the primary use case is as a deferred expression system, e.g. the typical flow is

conn = ibis.mapd.conn(....)
t = conn.table('tbl')
expr = t[t.colA > 50].group_by(t.colB).aggregate(t.count())...
output_df = expr.execute()

however, there are several cases where we find ourselves wanting a pre-existing SQL query to be the 'basis' of an expression - e.g.

query_str = 'select a, b, count(*) as ct from tbl where ... group by a,b having ...'
expr_query = expr_from_query_string(query) #(A 'pre-compiled' expression
expr2 = expr_query[(ct > 50) & (a == 'foo)] 

ideally should generate a query with a CTE for query_str i.e the above when compiled should return something like

WITH t0 as (
select a, b, count(*) as ct from tbl where ... group by a,b having ...'
) select * from t0 where t0.a == 'foo' and t0.ct > 50

[xmnlab]

not sure if it is what SQLClient.sql does. I am going to investigate that.