SQL Server's DateTime Data Type: The Missing Millisecond
Data accuracy is critical, especially when dealing with timestamps. However, SQL Server's datetime
type presents a common problem: the loss of milliseconds. This article explores why this occurs and offers solutions.
Consider this scenario: data is inserted using:
<code class="language-sql">INSERT INTO TESTTABLE (IntField, BitField, StringField, DateField) VALUES ('1', 1, 'hello', {ts '2009-04-03 15:41:27.378'});</code>
Retrieving the data with:
<code class="language-sql">select * from testtable with (NOLOCK)</code>
reveals a truncated DateField
: "2009-04-03 15:41:27.377". The last millisecond is gone.
The Root of the Problem
The datetime
data type's inherent limitation is the cause. SQL Server's datetime
only supports time precision to approximately 1/300th of a second (0, 3, or 7 milliseconds). Values outside these increments are rounded down. This explains the missing millisecond in our example.
Achieving Millisecond Precision
To maintain millisecond accuracy, alternative methods are necessary. There isn't a single perfect solution, but two common approaches are:
Numeric Fields: Store the timestamp as a numeric value (e.g., representing milliseconds since the epoch). This requires custom code to convert to and from a human-readable format.
String Representation: Store the timestamp as a string using a consistent format (e.g., 'YYYY-MM-DD HH:mm:ss.fff'). Optionally, include an approximate datetime
field for faster querying.
Both methods add complexity. Carefully evaluate the need for millisecond precision against the added design and maintenance overhead.
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