Legal limits on working while studying

I heard that EU/German students can't work more than 20 hours/week by law, although i have a German friend who works full time while studying.
What I know that if its a praktikum which is mandatory part of ur studies then u can work full time other than this its 20 hours max.
Any clue?

You can only work 19.25 hours per week during school and 38.5 hours during vacation time. Some Unis even set maximum per year hours for their studetns, i.e. 300 hours per calendar year.

Anything beyond that and you would have to get special permission from your school.

As a "Full time (non EU) Student" you can work: ( I have no clue how it works for EU Nationals)

(1) 180 Half / 90 Full days during the calender Year
(2) Max. of 19hrs/week during Semester and 40hrs/week during Semester break
(3) Can not work as "Werkstudent" more than 60 full days consecutively, otherwise your employer has to deal with Social Security guys.
(4) There are no limitations on working days if you work with University (in Bayern at least) so Number 1. is void but with Uni. you can work only 19hrs/week irrespective of during semester or during semester break.