Understanding and Working with SQL Server Timestamps and DateTimes

SQL Server provides several data types for storing date and time information. It’s crucial to understand the distinctions between these types, especially when dealing with TIMESTAMP (or ROWVERSION) and DATETIME columns, to avoid common pitfalls and ensure accurate data handling. This tutorial will clarify these differences and demonstrate how to work with each effectively.

What is a SQL Server Timestamp (ROWVERSION)?

Contrary to its name, the TIMESTAMP (now often referred to as ROWVERSION in newer SQL Server versions) data type does not store date or time information. Instead, it’s a binary number that SQL Server increments each time a row is modified. Its primary purpose is to track row versions, allowing you to detect changes and manage concurrency.

Key characteristics of TIMESTAMP/ROWVERSION:

Not a Date/Time: It does not represent a specific point in time.
Concurrency Control: Used to determine if a row has been modified since it was last read.
Binary Data: Stored as a binary number.
Automatic Increment: SQL Server automatically manages the incrementing value.

If you need to track when a row was created or last modified, using a dedicated DATETIME or DATETIME2 column is the correct approach.

Example:

-- Creating a table with a ROWVERSION column
CREATE TABLE MyTable (
    ID INT PRIMARY KEY,
    Name VARCHAR(50),
    RowVersion ROWVERSION
);

-- Inserting data
INSERT INTO MyTable (ID, Name) VALUES (1, 'Example');

-- The RowVersion is automatically populated by SQL Server
SELECT * FROM MyTable;

What are SQL Server DateTime Data Types?

SQL Server offers several data types specifically designed for storing date and time information, including:

DATETIME: Stores date and time values with an accuracy of approximately 3.33 milliseconds. It has a range of January 1, 1753, through December 31, 9999.
DATETIME2: Provides higher precision (up to 100 nanoseconds) and a wider range than DATETIME. It’s generally the preferred choice for new applications.
DATE: Stores only the date portion (year, month, day).
TIME: Stores only the time portion (hour, minute, second, fractions of a second).

Converting DateTime Values to Strings

Often, you’ll need to convert DATETIME values to string representations for reporting or display purposes. SQL Server provides the CONVERT and FORMAT functions for this.

Using CONVERT:

The CONVERT function allows you to convert a DATETIME value to a string with a specific style. The style code determines the format of the output string.

DECLARE @today DATETIME = SYSDATETIME();

-- Style 121: yyyy-mm-dd hh:mi:ss(24h)
SELECT CONVERT(VARCHAR(50), @today, 121); 
-- Output:  2023-10-27 14:30:00.123 (example)

-- Style 101: mm/dd/yyyy
SELECT CONVERT(VARCHAR(10), @today, 101);
-- Output: 10/27/2023 (example)

Refer to the Microsoft documentation for a complete list of style codes: https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql?view=sql-server-ver16

Using FORMAT (SQL Server 2012 and later):

The FORMAT function offers more flexible formatting options using .NET format strings.

DECLARE @today DATETIME = SYSDATETIME();

-- Using a .NET format string
SELECT FORMAT(@today, 'yyyy-MM-dd HH:mm:ss');
-- Output: 2023-10-27 14:30:00 (example)

Converting Strings to DateTime Values

You can also convert string representations to DATETIME values using CONVERT or CAST.

DECLARE @dateString VARCHAR(50) = '2023-10-27';

-- Using CONVERT with style 120
SELECT CONVERT(DATETIME, @dateString, 120);

-- Using CAST
SELECT CAST(@dateString AS DATETIME);

The success of this conversion depends on the format of the input string and the implicit or explicit style code provided.

Handling ROWVERSION/TIMESTAMP Data

As clarified earlier, ROWVERSION is not a date or time. If you need to compare row versions, you can simply compare the ROWVERSION values directly.

-- Comparing ROWVERSION values
SELECT * FROM MyTable WHERE RowVersion > '0x0000000012345678';

If you mistakenly treat ROWVERSION as a date, you will encounter errors or incorrect results.