consider the following relational schema that manages the telephobe bills of a mobile phone company.

CUSTOMER $($SSN$,$ Name, Surname, PhoneNum, Plan$)$

PRICINGPLAN $($Code$,$ ConnectionFee, PricePerSecond$)$

PHONECALL $($SSN$,$ Date, Time, CalledNum, Seconds$)$

BILL$($SSN$,$ Month, Year, amount$)$

write a trigger that at the end of each month discount the bills by $5\%$ per call to direct users of the t comapny $($that is to registered users in the table CUSTOMER$)$ if the total monthly amount of the bill excedds Rs $200.$