LISTING 4: Final Solution to the New Customers Problem
SELECT
  VY.ym,
  COUNT(*) - COUNT(VN.customerid) AS existing,
  COUNT(VN.customerid) AS new,
  COUNT(*) AS total
FROM Vymcusts AS VY
  LEFT OUTER JOIN Vnewcusts AS VN
    ON VY.ym = VN.min_ym
    AND VY.customerid = VN.customerid
GROUP BY VY.ym
ORDER BY VY.ym