Converting Strings to Datetime in SQL Server

In SQL Server, converting strings to datetime values is a common task, especially when working with data from various sources. In this tutorial, we will explore how to convert varchar columns or variables to datetime values in the desired format.

Understanding the CONVERT Function

The CONVERT function in SQL Server is used to convert data from one data type to another. When converting strings to datetime, it takes three arguments: the target data type (datetime), the expression to be converted (the string), and an optional style parameter that specifies the format of the input string.

Converting Varchar to Datetime

To convert a varchar string to a datetime value, you can use the CONVERT function with the datetime data type as the target. For example:

SELECT CONVERT(datetime, '2011-09-28 18:01:00', 120)

In this example, the style parameter 120 specifies that the input string is in the format YYYY-MM-DD HH:MI:SS. This is just one of many styles available for converting strings to datetime values.

Specifying the Output Format

Once you have converted a string to a datetime value, you may want to format it differently. You can do this by converting the datetime value back to a varchar using another style parameter. For example:

DECLARE @date datetime = CONVERT(datetime, '2011-09-28 18:01:00', 120)
SELECT CONVERT(varchar(30), @date, 105) + ' ' + CONVERT(varchar(8), @date, 108)

In this example, the style parameter 105 specifies that the output should be in the format DD-MM-YYYY, and the style parameter 108 specifies that the time part should be in the format HH:MI:SS.

Available Styles

SQL Server provides a range of styles for converting strings to datetime values. Some common styles include:

• 101: mm/dd/yyyy
• 102: yy.mm.dd
• 103: dd/mm/yyyy
• 104: dd.mm.yy
• 105: dd-mm-yyyy
• 106: dd mon yyyy
• 107: Mon dd, yyyy
• 108: hh:mi:ss
• 120: yyyy-mm-dd hh:mi:ss

You can find a full list of available styles in the SQL Server documentation or online resources such as W3Schools.

Best Practices

When converting strings to datetime values, it’s essential to specify the correct style parameter to avoid errors. Additionally, consider using the TRY_CONVERT function instead of CONVERT to handle invalid input strings more robustly.

By following these guidelines and examples, you should be able to convert varchar columns or variables to datetime values in SQL Server with ease.