Counting Differences within a Case-When - postgresql

Question

I'm trying to do some analytics on user activity, specifically how many users are still active, or at least logging in, over a period of time. However I have some conflicting numbers with the first months count which should just be the count of users that signed up during a month. To figure that out, my simple query is this.
SELECT count(user_id)
FROM users
WHERE date_part('year', member_since) = 2013
AND date_part('month', member_since) = 01
Hypothetically this returns '1,000' which I believe to be correct because of the simplicity. But if I do this...
SELECT
COUNT(CASE WHEN date_part('day', last_login - member_since) >= 0
THEN user_id END) days_0
FROM users
WHERE date_part('year', member_since) = 2013
AND date_part('month', member_since) = 01
...It will return a number less than 1,000. Theoretically this should return the same number as above because even if last_login is the same day as member_since that would be zero and should count those users. Both member_since and last_login are 'timestamp' types. I have a hunch that the difference could be users where last_login is the exact same as member_since, meaning that they signed up and never came back, but I'm not sure how I would test this. Is this a NULL issue? If so, how could I include that to get back to the count of '1,000'?

Answer 1

I would bet dollars to donuts that you nulls are what are causing the problem since they will always hit the else statement.
To correct for that, try this:
SELECT
COUNT(CASE WHEN last_login is null or date_part('day', last_login - member_since) >= 0
THEN user_id END) days_0
FROM users
WHERE date_part('year', member_since) = 2013
AND date_part('month', member_since) = 01;