SqlAlchemy Core and bare exists query

Question

I have faced a problem where I have to find if the data exists in the table using SqlAlchemy Core.

I think the best way to do this query is to use <

Answer 1

According to the documentation, exists applies to a Select object, and the following examples are provided:

# use on an existing select()
s = select([table.c.col1]).where(table.c.col2==5)
s = exists(s)

# construct a select() at once
exists(['*'], **select_arguments).where(criterion)

# columns argument is optional, generates "EXISTS (SELECT *)"
# by default.
exists().where(table.c.col2==5)

What is going wrong with your code then?

Well, from my understanding, EXISTS is not a directive by itself, so trying to execute an exists() will fail as it is not an executable clause.

To illustrate, you can try with a simple sqlite console:

• EXISTS(SELECT * from t); is an error
• SELECT EXISTS(SELECT * FROM t); yields 0 or 1 as expected.

How to fix your issue?

Wrap your exists() in a select() executable statement:

result = conn.execute(select([exists().where(cookie_table.c.cookie_id == cookie_id)]))

It should work as expected:

>>> print select([exists().where(users.c.name=='test')])
SELECT EXISTS (SELECT *
FROM users
WHERE users.name = :name_1)

Now, should you use exists or limit? Frankly I have no idea, and I'm not even sure the answer doesn't depend on your database engine...