Converting String to DATETIME in SQL Server: Best Practices and Methods

Introduction

Working with dates and times is a common requirement in many database applications. In SQL Server, converting strings to DATETIME can be straightforward when the string format matches one of the predefined date formats supported by SQL Server functions like CAST, CONVERT, or more recent functions such as TRY_PARSE. This tutorial will guide you through various methods for converting a string representation of a date and time into a DATETIME data type in SQL Server.

Understanding Date Formats

Before diving into conversion techniques, it’s essential to understand the format specifiers that SQL Server recognizes. Different cultures may represent dates differently (e.g., MM/DD/YYYY vs. DD/MM/YYYY). The ability to interpret these formats depends on using either specific date and time styles or leveraging locale settings.

Methods for String-to-DATETIME Conversion

1. Using CAST

The CAST function is a simple way to convert a string to DATETIME. However, it requires the input string to adhere strictly to one of SQL Server’s recognized formats.

-- Example: Convert '10/15/2008 10:06:32 PM' assuming MM/DD/YYYY format
SELECT CAST('10/15/2008 10:06:32 PM' AS DATETIME);

This approach is straightforward but lacks flexibility in handling different date formats.

2. Using CONVERT with Style Code

The CONVERT function provides more flexibility by allowing you to specify a style code that indicates the format of the input string.

-- Example using style code for US-format (MM/DD/YYYY hh:mi:ss:mmmAM)
SELECT CONVERT(DATETIME, '10/15/2008 10:06:32 PM', 101);

Here are some commonly used style codes:

101: mm/dd/yyyy
102: yy.mm.dd
103: dd/mm/yy
112: yyyymmdd
121: yyyy-mm-dd hh:mi:ss.mmm(24h)

You can refer to the official SQL Server documentation for a comprehensive list of style codes.

3. Using TRY_PARSE

TRY_PARSE is available starting from SQL Server 2012 and allows conversion using culture information, adding flexibility in handling different date formats across locales.

-- Example: Parse a string with US English settings
SELECT TRY_PARSE('10/15/2008 10:06:32 PM' AS DATETIME USING 'en-US');

This method attempts the conversion based on the provided locale and returns NULL if it fails, avoiding exceptions during conversion errors.

4. Using PARSE

Similar to TRY_PARSE, the PARSE function performs a conversion but throws an error if the parsing operation is unsuccessful. This can be useful when you need strict validation of date formats.

-- Example: Parse with error on failure
SELECT PARSE('10/15/2008 10:06:32 PM' AS DATETIME USING 'en-US');

5. Using SQL Server 2012 and Later Features

Starting from SQL Server Denali (version 110), the FORMAT function allows custom formatting using .NET’s format strings, but it requires a valid date input first.

-- Example: Format a DATETIME to a string with custom formatting
DECLARE @d DATETIME = '2008-10-13 18:45:19';
SELECT FORMAT(@d, N'MMM-dd/yyyy HH:mm:ss');

Best Practices

Standardize Input Formats: Encourage users to input dates in a consistent format using date picker controls rather than free text fields.
Validate Inputs: Implement validation logic before conversion to ensure that the string adheres to an expected format.
Choose Appropriate Methods: Use CONVERT with style codes for known formats, and use TRY_PARSE or PARSE when dealing with potentially varying locale-specific inputs.
Error Handling: Utilize TRY_PARSE in scenarios where you want graceful handling of parsing errors by returning NULL instead of throwing exceptions.

Conclusion

Converting strings to DATETIME in SQL Server can be achieved through various methods, each suitable for different scenarios depending on the format and locale considerations. By leveraging these techniques effectively, you can ensure robust date-time handling within your database applications.