sql - MySQL group by without column name -

i have table contains user infos.

mysql> desc accounts; +-------------+------------+------+-----+---------+----------------+ | field       | type       | null | key | default |          | +-------------+------------+------+-----+---------+----------------+ | cid         | int(11)    | no   | pri | null    | auto_increment | | username    | text       | yes  |     | null    |                | | password    | text       | yes  |     | null    |                | | mysignature | text       | yes  |     | null    |                | | is_admin    | varchar(5) | yes  |     | null    |                | +-------------+------------+------+-----+---------+----------------+ 5 rows in set (0.01 sec) 

normal group statement follows 1 or more column names below:

select * accounts group cid; 

just example, in general group by works aggregate functions sum().

i have found follows expression without column names cant understand:

mysql> select username accounts group now(); +----------+ | username | +----------+ | admin    | +----------+ 1 row in set (0.00 sec) 

i new in mysql. how query work ? thanks.

in fact it's expression. compares expression results instead of column values grouping.