oracle query

Already answered question

https://www.experts-exchange.com/questions/27763628/oracle-query-in-a-loop.html.

Having problems

Basically user cannot enter any reason_codes if there is already an existing schedule for that schedule_date and time.

select schedule_date,payroll_id,start_time,
       start_ampm,total_hours,end_time,end_ampm
  from dept_staff
 where payroll_id = 'ZZW0024100'
 and schedule_date = '01-oct-2012'

SCHEDULE_DATE|PAYROLL_ID|START_TIME|START_AMPM|TOTAL_HOURS|END_TIME|END_AMPM
10/1/2012|ZZW0024100|800|A|800|400|P

I entered the same start time, start_ampm, but changed the total_hours to 7.0 instead of 8.0 as previous testing. Did not give me error. Its not logical to enter any time if that schedule exists. I tried to change my query from count(*) from the previous answered question to the following

  select count(*) into v_count
  from dept_staff
 where payroll_id = :dept_staff.payroll_id
   and schedule_date = :dept_staff.schedule_date
   and start_time = :dept_staff.start_time
   and end_time = :dept_staff.end_time
   and start_ampm = :dept_staff.start_ampm
   and end_ampm = :dept_staff.end_ampm;
Thats why for my second test it did not stop from entering. Is there a way to modify the query to make sure the user will not be able to enter any thing between those time schedule that is present for that date?
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