Select count(*) from result query

Question

I need help from you, this is my sql query:

select count(SID) 
from Test 
where Date = \'2012-12-10\' 
group by SID

this is my result:

Answer 1

This counts the rows of the inner query:

select count(*) from (
    select count(SID) 
    from Test 
    where Date = '2012-12-10' 
    group by SID
) t

However, in this case the effect of that is the same as this:

select count(distinct SID) from Test where Date = '2012-12-10'