We dont have grocery bagers. I assume you mean people whos sole income are tips? Thats not legal here. You can tip the pizza guy and waiter, but they are employees getting a paycheck from their employers. Tips are a bonus for a job well done and unrelated to them making a living.
Part time with a fixed schedule works the same way.
With flexible hours the average working hours per day are calculated. So if you work for 40 hours a month, a sick day gets you around 2 hours of sick leave.
edit: I think I got you now, you mean day labor, who get payed for how long they can work for changing employers?
I had to google for that, those are "Unständig Beschäftigte". Those are people who are employed for less then a week. They dont get sick leave. I couldnt tell you an example where it is allowed to hire someone like that. Even field workers/harvest helper get a normal contract with predefined working hours.