SQLAlchemy and empty IN clause

Question

I found out that SQLAlchemy translates

db.query(...).filter(A.id.in_(ids))

into

SELECT ...
FROM a
WHERE a.id != a.id
         


        

Answer 1

Be aware what you are asking for:

• Only if the value of A.id is comparable any comparison can actually succeed. The non-existing value is not comparable to anything, all comparisons will result in a non-existing value which in turn is evaluated as False. That is, if A.ID is NULL then A.ID == anything is False and A.ID != anything is also False: A.ID == A.ID || A.ID != A.ID is False if A.ID is NULL.
• The IN-clause with an empty sequence asks if the value is part of the empty list. The non-existing value is part of no list, not even the empty one.
• Hence what you are asking for is some variation of IS NOT NULL and something that is part of nothing. This is a condition that has to be checked for. The non-existing value is not something; only some value that is not NULL can not be be a member of the empty list...
• As sqlalchemy is clever about the fact that this is probably not the way you want to express this condition, it gives a warning. You should probably drop the IN-clause if the sequence is empty.

For a concrete example this sqlfiddle

For a more philosophical approach see What is the nature of void