Converting Date Strings to Datetime in SQL Server 2008 using T-SQL

Introduction

When working with date and time data in a relational database, it is essential to have the correct data type to ensure accurate calculations, sorting, and filtering. In SQL Server 2008, one common issue is converting string representations of dates into datetime format. This article will explore how to convert date strings to datetime using T-SQL.

Understanding Date and Time Data Types in SQL Server

Before we dive into the conversion process, it is crucial to understand the available date and time data types in SQL Server:

• datetime: Represents a date and time within the 100-year range of January 1, 1753, through December 31, 2257.
• date: Represents a date within the range of January 1, 1601, through December 31, 9999.

SQL Server 2008 uses the datetime data type by default. This is because the datetime format stores both date and time components together.

Converting Date Strings to Datetime

To convert a date string into datetime format using T-SQL, you can utilize the CONVERT() function with the corresponding style arguments.

The general syntax for converting a value from one data type to another is:

CONVERT(data_type, expression[, style])

In this context, we will use DATETIME as the target data type and an integer style argument to specify the conversion format. The available styles are:

• 101: The system default date and time format for SQL Server.
• 103: A style similar to mm/dd/yyyy.
• 104: A style similar to dd-mm-yyyy.

Example Code

DECLARE @S1 VARCHAR(20) = '1-1-2017';
DECLARE @S2 VARCHAR(20) = '11/1/2017';
DECLARE @S3 VARCHAR(20) = '1/11/2017';
DECLARE @S4 VARCHAR(20) = '11/11/2017';

SELECT CONVERT(DATETIME, @S1 , 104) AS S1,
       CONVERT(DATETIME, @S2 , 103) AS S2,
       CONVERT(DATETIME, @S3 , 101) AS S3,
       CONVERT(DATETIME, @S4 , 102) AS S4;

In this code snippet, we declare four variables holding different date strings in various formats. We then use the CONVERT() function to convert these strings into datetime format.

Notes on Conversion Styles:

• The conversion style is specified using an integer argument.
• Using 104 as the style argument ensures that the conversion uses the same date and time format, regardless of how it was originally stored in the input string. This is particularly useful when dealing with date strings that might have been entered manually or through various data entry interfaces.

Handling Different Date Formats

The example code above assumes a specific set of date formats for ease of illustration. However, SQL Server can handle more complex date formats:

• dd/mm/yyyy: Use the style argument 103.
• mm/dd/yyyy: Use the style argument 101.
• yyyy-mm-dd: Use the style argument 120.

When dealing with date strings in different formats, it’s a good practice to test the conversion using various styles until you find the one that works best for your specific use case.

Best Practices

To avoid potential errors when working with date and time data in SQL Server:

• Always verify the accuracy of date calculations by checking the results against known values or by comparing them to other data sources.
• Use the GETDATE() function to retrieve the current system date, which helps ensure accurate comparisons and calculations.

Conclusion

Converting date strings to datetime format is an essential skill when working with SQL Server. By understanding how to use T-SQL’s CONVERT() function with the corresponding style arguments, you can easily handle various date formats and store them in a consistent datetime data type for further analysis or processing.

Last modified on 2025-02-11