How does works "group by" correct

[SoftIl]

Hi all,
I've got some problems with "group by":

1)
If I try this:
select * from myTable group by date
the "group by" statement hasn't an effect.

If I write this
select *,count(any column) from myTable group by date
it works correct.

Why is it so? In no documentation I could find this, that I have to use
"count()" if I'd like to use "group by".

2)
How can I use "group by" for more than one column?
This statement works not correct. I get all records return:
select *,count(any column) from myTable group by date,ohrnummer

Thanks for your help.
Carsten