When does COUNT(*) return NULL?

Question

Hello I have a colleague who always writes ISNULL(COUNT(*),0), but I always thought that COUNT(*) could never return NULL.

But then I searched the interwebs and my findings allowed me to write this little piece of code:

create table t1 (
    val1 varchar(50),
)

select count(*) from t1
where val1 like 'abc'
group by val1

Are there any other cases when COUNT(*) returns NULL?

Answer 1

It doesn't return NULL. The GROUP BY in your example makes it return no rows at all, which is not the same as a NULL in a column.

Answer 2

That example doesn't return NULL. It returns no rows at all because of the GROUP BY on an empty set.

COUNT(*) cannot return a NULL. So the ISNULL is unnecessary.