WITH MaxPerDay as (
  SELECT
  CustomerID,
  CONVERT(VARCHAR, STARTDATE, 105) as 'Day',
  MAX(SCORE) as 'MaxScore'
  FROM
Interactions
  GROUP BY CustomerId, CONVERT(VARCHAR, STARTDATE, 105)
)
SELECT
MaxPerDay.CustomerID,
SUM(MaxPerDay.MaxScore)
from MaxPerDay
GROUP BY MaxPerDay.CustomerID
order by SUM(MaxPerDay.MaxScore) desc