title: PLSQL-estimate specific hours of business days between two dates
body: <p>I have a query that uses a difference between two dates in order to find the duration used somewhere else in the query'
  Firstly, i want to exclude no business days, and secondly, from the remaining ones to use only a predefined working hour period (i.e 08:00 - 20:00).
  This working hour period will be the same for Monday-Wednesday-Friday (08:00-20:00) and 09:00-17:00 for Tuesday and Thursday. </p>