title: How to count the combnation which consists of two values both from different tables?
body: <p>I have the next schema:
  <a href="https://i.stack.imgur.com/V1W6i.png" rel="nofollow noreferrer">the schema</a></p>
  <p>I need to identify those car owners who are always serviced by the same mechanic and output the names of the mechanic and his regular customer.</p>
  <p>I have tried this code, but it only gives me the combinations, how can I count this combinations and print the most frequent?</p>
  <pre><code>select cars_license_plate,mechanics_personnel_number,count(*) from orders where cars_license_plate in
      (
          select license_plate from cars where drivers_license_number in
          (
              select license_number from drivers where full_name in(select distinct full_name from drivers)
          )
      ) group by cars_license_plate, mechanics_personnel_number
  ;
  </code></pre>