Question

I’ve decided that I’ll use 8601 datetimes for all the datetimes that I return from my app. Suddenly, in one particular proc, getdate() isn’t returning a datetime with a T in the middle. I should also mention that I’m converting the set containing a datetime to XML using FOR XML PATH. Typically when I convert a table containing datetime to xml I get 8601 formatted dates. But in one case, I’m not.

select (cast(getdate() as datetime)) -- returns 2010-01-25 10:13:46.033

So I directly converted it like so:

select convert(datetime, getdate(), 126) -- returns 2010-01-25 10:14:35.923

But if I cast it to an nvarchar I get the T!!

SELECT CONVERT(NVARCHAR(30), GETDATE(), 126) -- returns 2010-01-25T10:15:29.633

What’s even stranger to me is that if I select several versions of this with a union, the T version disappears. But selecting without the union, the T version (last one) remains.

-- returns 4 rows of 2010-01-25 10:15:57.333
select getdate() union all 
select (cast(getdate() as datetime))  union all
select convert(datetime, getdate(), 126)    union   all
SELECT CONVERT(NVARCHAR(30), GETDATE(), 126) 

I really have no idea what could cause this to happen. I thought 8601 dates were locale-independent, so I don’t think it’s anything like that.

Reference (“yyyy-mm-ddThh:mi:ss.mmm” for a 126): http://msdn.microsoft.com/en-us/library/ms187928.aspx

Answer 1

You are mixing up datetime values with formatted strings.

The first two examples doesn’t return the date formatted in any way at all, it’s just a datetime value. How the value is formatted into text is decided by how you display the value after getting it from the database. If you lived in a different country so that your default culture settings were different, the date could for example be displayed as 1/25/2010 10:14 AM instead.

In the second example the format parameter (126) is ignored, as there is no formatting or parsing involved when converting from a datetime value to a datetime value.

The third example formats the datetime value into a string before it’s returned from the database, that’s why you get it in the format that the database uses.

If you use a union with difference data types, it’s the type precedence that decides the type of the result. The datetime type has higher precedence than nvarchar, so it attempts to convert the nvarchar values to datetime.