SQL Server : datetime comparison ignoring few milliseconds [duplicate]

Question

I noticed that 2 datetimes are considered equal if one is

2010-12-31 15:13:48.000 

and the other

2010-12-31 15:13:48.001.

I thought in the beginning that it was rounding up or down to nearest full seconds but it turned out

2010-12-31 15:13:48.002 

registers the difference correctly.

Any idea why this happens?

Answer 1

The datetime data type is accurate to 3 milliseconds (0.0033 seconds), so your results make sense. Values are rounded to 0.000, 0.003, and 0.007. So, your first example is rounded to: "2010-12-31 15:13:48.000" and the second to "2010-12-31 15:13:48.003".

You can read about this in the documentation.