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

You're spoilt for choice in terms of options here. If you are using Sybase or SQL Server 2008 you can create variables of type date and assign them your datetime values. The database engine gets rid of the time for you. Here's a quick and dirty test to illustrate (Code is in Sybase dialect):

declare @date1 date
declare @date2 date
set @date1='2008-1-1 10:00'
set @date2='2008-1-1 22:00'
if @date1=@date2
    print 'Equal'
else
    print 'Not equal'

For SQL 2005 and earlier what you can do is convert the date to a varchar in a format that does not have the time component. For instance the following returns 2008.08.22

select convert(varchar,'2008-08-22 18:11:14.133',102)

The 102 part specifies the formatting (Books online can list for you all the available formats)

So, what you can do is write a function that takes a datetime and extracts the date element and discards the time. Like so:

create function MakeDate (@InputDate datetime) returns datetime as
begin
    return cast(convert(varchar,@InputDate,102) as datetime);
end

You can then use the function for companions

Select * from Orders where dbo.MakeDate(OrderDate) = dbo.MakeDate(DeliveryDate)