Query on datetime fields with milliseconds gives wrong result in SQL Server

Question

I'm running into an odd bug using datetime fields in SQL Server 2005. The datetime field shows up with millisecond-level accuracy, but it looks like the milliseconds are not always used. Here's my test query:

SELECT col1, YEAR(col1) AS yr, MONTH(col1) AS mn, DAY(col1) AS dy
FROM mytable
WHERE col1 >= '2009-12-31 00:00:00.0' AND col1 <= '2009-12-31 23:59:59.999'
ORDER BY col1

In my results I get:

 col1                       |  yr  | mn | dy
----------------------------+------+----+----
 2009-12-31 00:00:00:00.000 | 2009 | 12 | 31
 2010-01-01 00:00:00:00.000 | 2010 |  1 |  1

The problem is that I got the 2010-01-01 date, even though that shouldn't be less than or equal to "2009-12-31 23:59:59.999". But if I change the query to use "2009-12-31 23:59:59.998" it works OK (no 2010 datetimes are returned).

Is this a bug, or is this just how SQL Server works? If this is how it works, is there some reason for it? I ran into this migrating some queries from MySQL, where this works as expected (even though MySQL doesn't even store the milliseconds!).

Answer 1

SQL Server stores time part as number of 1/300 second long ticks from the midnight.

23:59:59.999 gets rounded to the nearest tick which happens to be 00:00:00.000 of the next day.

SELECT  CAST(CAST('2009-12-01 00:00:00.000' AS DATETIME) AS BINARY(8)),
        CAST(CAST('2009-12-01 23:59:59.997' AS DATETIME) AS BINARY(8)),
        CAST(CAST('2009-12-01 23:59:59.999' AS DATETIME) AS BINARY(8))



0x00009B8F 00000000    0x00009B8F 018B81FF    0x00009B90 00000000

In the first value, the date part, 0x9B8F (39823) is the number of days since Jan 1st, 1900, and the time part, 0, is the number of ticks since midnight.

In the second value, 0x018B81FF (25919999, or 24 * 60 * 60 * 300 - 1) is the maximal possible number of ticks since midnight.

Finally, the third value has the 0 in the time part and the date part increased by one.