What's a good way to check if two datetimes are on the same calendar day in TSQL?

Question

Here is the issue I am having: I have a large query that needs to compare datetimes in the where clause to see if two dates are on the same day. My current solution, which suck

Answer 1

Eric Z Beard:

the activity date is meant to indicate the local time zone, but not a specific one

Okay - back to the drawing board. Try this:

where t.TheDateINeedToCheck BETWEEN (
    dateadd(hh, (tz.Offset + ISNULL(ds.LocalTimeZone, 0)) * -1, @ActivityDate)
    AND
    dateadd(hh, (tz.Offset + ISNULL(ds.LocalTimeZone, 0)) * -1, (@ActivityDate + 1))
)

which will translate the @ActivityDate to local time, and compare against that. That's your best chance for using an index, though I'm not sure it'll work - you should try it and check the query plan.

The next option would be an indexed view, with an indexed, computed TimeINeedToCheck in local time. Then you just go back to:

where v.TheLocalDateINeedToCheck BETWEEN @ActivityDate AND (@ActivityDate + 1)

which would definitely use the index - though you have a slight overhead on INSERT and UPDATE then.