Find free time slots per day

Hello I need support for a small project to find the free time slots per day and user. The working times of employee are stored in a mysql table as it is at attached document. Every employee has to work between hours "gs" and "ge". The expected max. hours are per employee are in "abszsollstd". If employee worked between gs and ge per day less than abszsollstd then I want to get the possible time slot per employee and day to catch up the expected working hours. So what I am expecting is to have an mysql query which show me the possible time slot if the employee has worked les than expected hours. Please note: each employee can enter more than one working time per day. I want to see only the timeslots for each entered day and employee. So please find first how many hours has an employee entered per day...