Excellent summary from Jeff Smith on how best to group data by time periods.
Quickhint:
If in doubt, the ‘go-to’ technique is to use
GROUP BY dateadd(month, datediff(month, 0, SomeDate),0)
replacing ‘month’ with any of:
- year
- quarter
- month
- dayofyear
- day
- week
- hour
- minute
- second
- millisecond
- microsecond
- nanosecond
Read the article for a great write up on the whys and wherefores and as always your mileage may vary – testing the various techniques with your own data is the only sure way of getting the performance you need.
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