Create table Bills
(
  BillNo INT NOT NULL,
  MeterNo INT NOT NULL,
  FirstDate datetime NOT NULL,
  FirstDateReading INT NOT NULL,
  CurrentDate as MAX(FirstDate) CurrentDateReadng INT NOT NULL,
  LastDate as DATEADD(m,-1,CurrentDate) LastDateReading INT NOT NULL,
  Usageinm3 as (CurrentDateReadng-LastDateReading),
  Usageinlitres as (Usageinm3 * 1000), TotalBill as (UsageinLitres * 3) Primary Key (BillNo),
  Foreign Key (MeterNo) References MeterReading(MeterNo)
);