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Asked: 2026-05-10T13:31:06+00:00

Here is the issue I am having: I have a large query that needs

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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 sucks, is to send the datetimes into a UDF to convert them to midnight of the same day, and then check those dates for equality. When it comes to the query plan, this is a disaster, as are almost all UDFs in joins or where clauses. This is one of the only places in my application that I haven’t been able to root out the functions and give the query optimizer something it can actually use to locate the best index.

In this case, merging the function code back into the query seems impractical.

I think I am missing something simple here.

Here’s the function for reference.

if not exists (select * from dbo.sysobjects                where id = object_id(N'dbo.f_MakeDate') and                              type in (N'FN', N'IF', N'TF', N'FS', N'FT'))   exec('create function dbo.f_MakeDate() returns int as           begin declare @retval int return @retval end') go  alter function dbo.f_MakeDate (     @Day datetime,      @Hour int,      @Minute int ) returns datetime as  /*  Creates a datetime using the year-month-day portion of @Day, and the  @Hour and @Minute provided  */  begin  declare @retval datetime set @retval = cast(     cast(datepart(m, @Day) as varchar(2)) +      '/' +      cast(datepart(d, @Day) as varchar(2)) +      '/' +      cast(datepart(yyyy, @Day) as varchar(4)) +      ' ' +      cast(@Hour as varchar(2)) +      ':' +      cast(@Minute as varchar(2)) as datetime) return @retval end  go

To complicate matters, I am joining on time zone tables to check the date against the local time, which could be different for every row:

where  dbo.f_MakeDate(dateadd(hh, tz.Offset +      case when ds.LocalTimeZone is not null      then 1 else 0 end, t.TheDateINeedToCheck), 0, 0) = @activityDateMidnight

[Edit]

I’m incorporating @Todd’s suggestion:

where datediff(day, dateadd(hh, tz.Offset +      case when ds.LocalTimeZone is not null      then 1 else 0 end, t.TheDateINeedToCheck), @ActivityDate) = 0

My misconception about how datediff works (the same day of year in consecutive years yields 366, not 0 as I expected) caused me to waste a lot of effort.

But the query plan didn’t change. I think I need to go back to the drawing board with the whole thing.

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1 Answer

  1. This is much more concise:

    where    datediff(day, date1, date2) = 0
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