Specify multiple alternate Join conditions using conditional expression with SQLalchemy

Question

I'm joining two tables in SQLalchemy. If a certain condition is met I want to join on column 'C' in the right table otherwise I want to join on column 'D' in the right table.

With raw SQL I would use a CASE statement. I tried to implement this in SQLalchemy but got this error:

TypeError: 'bool' object is not iterable

Here's how I would write the statement in raw SQL:

SELECT 
t1.A, 
t2.B
FROM t1
JOIN t2 ON CASE
    WHEN t2.E = '1' THEN t2.C
    ELSE t2.D
END LIKE t1.CD || %

And how I tried to write it in SQLalchemy:

select([t1.columns.A, t2.columns.B]).select_from(
   t1.join(t2, case([t2.columns.E == '1', t2.columns.C], 
   else_=t2.columns.D).like(t1.columns.CD + '%')))

Answer 1

The syntax of my case statement was wrong. The condition should be a tuple surrounded in brackets. Instead of:

case([t2.columns.E == '1', t2.columns.C], else_=t2.columns.D)

It should be:

case([(t2.columns.E == '1', t2.columns.C)],  else_=t2.columns.D)

Answer 2

I don't think you can do a like statement as a join condition. You can do a cartesian product join and do a where statement with a like in it.

SELECT t1.A, t2.B
FROM t1, t2
where CASE WHEN t2.E = '1' THEN t2.C ELSE t2.D END LIKE t1.CD||'%'