How do I manage dates in a stored procedure

Question

Hey folks,
I have a table like this:
STUDENT
ID INT
NAME VARCHAR
REGISTRATION_DATE DATETIME
create table student (ID int, NAME varchar, REGISTRATION_DATE datetime)
Insert into student (ID, NAME, REGISTRATION_DATE)
values
(1, 'NAME1', '2014-11-10 06:00:00.000'),
(2, 'NAME2', '2014-11-11 06:00:00.000'),
(3, 'NAME3', '2014-11-12 06:00:00.000'),
(4, 'NAME4', '2014-11-13 06:00:00.000'),
(5, 'NAME5', '2014-11-14 06:00:00.000')
I have the following store procedure
CREATE PROCEDURE TRGetMsgTksByBIdDIdPIdAMsreDte
@REGISTRATION_DATE DATETIME = NULL
AS
SET NOCOUNT ON
SELECT *
FROM STUDENT
WHERE REGISTRATION_DATE = @REGISTRATION_DATE
RETURN 0
GO
My problem has to do with date formats and specifically with localizations (sometimes it is saved as MM-DD-YY, others DD-MM-YY, and it does not correspond with the parameter. How can I solve this problem WITHOUT having to apply a format conversion inside
the WHERE clause? (because when I use a function inside the WHERE, the index created for this column is invalidated and the query takes longer and longer when the DB gets bigger and bigger)
Thanks,
Marco

Answer 1

Hello Marco,
What is important here is the source of your table's data.  When loading the table with data, your source date values must have a clean, consistent format.  
For example, if the following two rows represent December 7th, depending on your SQL Server installation / localization, only ONE row will be correct.  The other will be inserted as July 12th.  
INSERT INTO student (ID, NAME, REGISTRATION_DATE)
VALUES
(1, 'Name1', '12/7/2014'),
(2, 'Name2', '7/12/2014')
In other cases, the INSERT statement may fail altogether because the date string won't "fit" the localization settings.  For example "25/12/2014" won't fit into a USA-localized SQL Server (there is no such thing as month 25).  "US
English" is SQL Server's default localization, by the way, and requires dates to be entered with any variation of "month/day/year," or better, in the international format "year/month/day."
Once your data is cleanly loaded into the table, you can use various string formats in your stored procedure's input parameter, but keep in mind your SQL Server's localization.  All of the following execution statements will work for a query of December
7th on a US SQL Server installation:
exec TRGetMsgTksByBIdDIdPIdAMsreDte '12/7/2014'
exec TRGetMsgTksByBIdDIdPIdAMsreDte '2014-12-7'
exec TRGetMsgTksByBIdDIdPIdAMsreDte 'December 7, 2014'
TIP:
Be careful when querying fields with the "datetime" format.  In your stored procedure, records in the table might be filtered out if they include time information, and if your input parameter fails to specify the exact time.
A better way might be to change your input parameter datatype to "date" instead of "datetime," then inside the stored procedure, transfer the original input parameter value to start and end datetimes to match the table's datatype.
Example:
CREATE PROCEDURE TRGetMsgTksByBIdDIdPIdAMsreDte
@REGISTRATION_DATE DATE
AS
SET NOCOUNT ON
DECLARE @DATETIME_START DateTime,
@DATETIME_CUTOFF DateTime
SELECT @DATETIME_START = @REGISTRATION_DATE -- SQL Server will implicitly expand the input's "date" format into "datetime" type representing midnight of the given date.
SELECT @DATETIME_CUTOFF = DATEADD(day, 1, @REGISTRATION_DATE) -- the cutoff for the next day
SELECT *
FROM STUDENT
WHERE REGISTRATION_DATE >= @DATETIME_START
AND REGISTRATION_DATE < @DATETIME_CUTOFF
RETURN 0
GO
The above code will ensure that any index on "REGISTRATION_DATE" is being used (field REGISTRATION_DATE and @DATETIME_START, @DATETIME_CUTOFF are of the same datatype).  The above code will also ensure that if records are stored with time
information (eg. '12/7/2014 10:30:00'), that they don't get filtered out when the input parameter @REGISTRATION_DATE = '12/7/2014' (implicitly midnight).
Hope this helps.
Tuan