Question

Firstly, this is not asking In SQL, what's the difference between count(column) and count(*)?.

Say I have a users table with a primary key user_id and another field logged_in which describes if the user is logged in right now.

Is there a difference between running

SELECT COUNT(user_id) FROM users WHERE logged_in=1

and

SELECT COUNT(logged_in) FROM users WHERE logged_in=1

to see how many users are marked as logged in? Maybe a difference with indexes?

I’m running MySQL if there are DB-specific nuances to this.

Answer 1

In MySQL, the count function will not count null expressions, so the results of your two queries may be different. As mentioned in the comments and Remus’ answer, this is as a general rule for SQL and part of the spec.

For example, consider this data:

user_id   logged_in
1         1
null      1

SELECT COUNT(user_id) on this table will return 1, but SELECT COUNT(logged_in) will return 2.

As a practical matter, the results from the example in the question ought to always be the same, as long as the table is properly constructed, but the utilized indexes and query plans may differ, even though the results will be the same. Additionally, if that’s a simplified example, counting on different columns may change the results as well.

See also this question: MySQL COUNT() and nulls