Understanding datetime formats in SQL Server

Question

I've dealt with MS SQL Server datetime types for a long time but never thought why the following is happening:

1. I query a table that contains a smalldatetime column. This smalldatetime is always returned in the format yyyy-MM-dd hh:mm:ss
2. Now I write a different query on which I want to apply a smalldatetime filter in the WHERE clause, something like WHERE TimeStamp >= 'yyyy-MM-dd hh:mm:ss'
3. SQL Server retrieves an error and tells me that was not possible to convert that nvarchar to a valid smalldatetime

It appears that it only works if I change the specified format and I write it using the european format, like WHERE TimeStamp >= 'dd-MM-yyyy hh:mm:ss'. Why is SQL Server showing me the dates in a format that is not covertable or valid when applied back to itself? I don't have any problem in changing the date format when writing queries, but I want to play with these dates at an application level (Java-JDBC app) and I don't want to be applying date format changes all the time... Could anyone explain me why this is happening and if there is any way to solve it at a DB level? Thanks!!

Edit: Please see the screenshot of the error in Management Studio below. enter image description here

Answer 1

The only truly safe formats for DATETIME/SMALLDATETIME in SQL Server are:

yyyyMMdd
yyyyMMdd hh:nn:ss[.mmm] -- or up to [.mmmmmmm] for datetime2 etc.
yyyy-MM-ddThh:nn:ss[.mmm] -- or up to [.mmmmmmm] for datetime2 etc.
----------^ yes, that T is important!

Anything else is subject to incorrect interpretation by SQL Server, Windows, the provider, the application code, end users, etc. For example, the following always breaks:*

SET LANGUAGE FRENCH;
SELECT CONVERT(DATETIME, '2013-11-13');

Result:

Le paramètre de langue est passé à Français.
Msg 242, Level 16, State 3, Line 2
La conversion d'un type de données varchar en type de données datetime a créé une valeur hors limites.

Just changing the language (which any of your user sessions can do) forced SQL Server to interpret that as YYYY-DD-MM instead of YYYY-MM-DD. Similar things can happen with setting like DATEFORMAT. But these settings are literally ignored when using the above two formats.

Always, always, always use one of the above two formats. If you are passing a variable as a string, stop doing that. If you can't, check to make sure it passes ISDATE() first. If you are letting people type any date string into a form field, stop doing that, too. Use a date-picker or calendar control and dictate the format of the string before you pass it to SQL Server. Well, depending on the language, just keep it as a datetime value and don't convert it to a string at all.

• I can't reproduce your problem at all, btw...

Please read this post:

• Bad Habits to Kick : Mis-handling date/range queries

_{There is an exception: SELECT CONVERT(DATE, 'yyyy-mm-dd'); will not break. But I err on the side of consistency rather than using a format only in the one place where I know it doesn't break, and having to use a safer format everywhere else.}

• Paul White: Comments are not for extended discussion; this conversation has been moved to chat.

Answer 2

According to SQL Server Books Online Write International Transact-SQL Statements you could wrap your date strings like so:

{ ts 'yyyy-mm-dd hh:mm:ss[.fff] '}

Example:

{ ts '1998-09-24 10:02:20' }

This might be easier to accomplish for you because instead of inserting a T in the middle, you are just concatenating strings at the beginning and the end.

One caveat: you have no control over the precise type when using this syntax; the result is always datetime. To achieve a smalldatetime result (as in the question), you would need to use a convert (or cast). In which case, you might as well use just convert (with the correct style parameter) in the first place.