Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 13: User-Defined Functions
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unlikely to return us any data — even if it happened on the same day (it would have had to have happened
within in the same minute for a smalldatetime, within a millisecond for a full datetime field,
and potentially down to as close as 100 milliseconds for datetime2).
The typical solution is to convert the date to a string and back in order to truncate the time information,
and then perform the comparison.
It might look something like:
SELECT *
FROM Orders
WHERE CONVERT(varchar(12), OrderDate, 101) = CONVERT(varchar(12), GETDATE(), 101)
It is certainly worth noting that you could also do this by simply casting the value of @Date to the
date data type. I’ve chosen to use CONVERT here just to show a more backward-compatible way of
truncating dates (SQL Server 2005 and earlier did not support the date data type).
This time, we will get back every row with today’s date in the OrderDate column, regardless of what
time of day the order was taken. Unfortunately, this isn’t exactly the most readable code. Imagine you had
a large series of dates you needed to perform such comparisons against — it can get very ugly indeed.
So now let’s look at doing the same thing with a simple user-defined function. First, we’ll need to create
the actual function. This is done with the new CREATE FUNCTION command, and it’s formatted much
like a sproc. For example, we might code this function like this:
CREATE FUNCTION dbo.DayOnly(@Date date)
RETURNS date
AS
BEGIN
RETURN @Date;
END
where the date returned from GETDATE() is passed in as the parameter and the task of converting the
date is included in the function body and the truncated date is returned.
Note that the preceding version is a SQL Server 2008 compatible version, relying on the coercion into the
parameter’s date data type to truncate the time. If you wanted to do a truncation like this in SQL Server
2005 (as we did with the query-based example), you would need to use the CONVERT function as we did
before. For example:
CREATE FUNCTION dbo.DayOnly(@Date datetime)
RETURNS varchar(12)
AS
BEGIN
RETURN CONVERT(varchar(12), @Date, 101);
END
To see this function in action, let’s re-format our query slightly:
SELECT *
FROM Orders
WHERE dbo.DayOnly(OrderDate) = dbo.DayOnly(GETDATE());