Explanation:

Comprehensive Detailed Explanation
Step 1: Requirement
We need to compare the average miles per trip for:
Statutory holidays (when IsHoliday = 1)
Non-statutory holidays (when IsHoliday = 0)
Step 2: Formula for average miles per trip
Average miles per trip = total miles ÷ number of trips
Total miles # SUM(t.tripDistance)
Number of trips # COUNT(t.tripID)
So the calculation is:
(SUM(t.tripDistance) / COUNT(t.tripID)) AS MilesPerTrip
Step 3: Grouping
We need a comparison by holiday status.
So we must group the results by:
GROUP BY d.IsHoliday
This ensures we get two rows: one for IsHoliday = 1 and one for IsHoliday = 0.
Step 4: Final Query
SELECT
d.IsHoliday,
(SUM(t.tripDistance) / COUNT(t.tripID)) AS MilesPerTrip
FROM DimDate d
INNER JOIN Trips t ON d.DateID = t.DateID
GROUP BY d.IsHoliday;
Why This is Correct
The formula ensures average miles per trip.
Grouping ensures comparison between holidays vs non-holidays.
Efficient aggregation, minimal computation.
References
Aggregate functions in T-SQL
GROUP BY clause