CAST vs. CONVERT in T-SQL: When Should I Use Which Function?

T-SQL data type conversion: choice between CAST and CONVERT

In T-SQL, CAST and CONVERT functions are used to convert data from one data type to another. Although both functions achieve similar results, there are subtle differences between them that may affect your choice.

General Guidelines for CAST and CONVERT

• CONVERT is a SQL Server-specific function that provides greater flexibility, allowing you to format dates and use additional parameters.
• CAST follows the ANSI-SQL standard and is more concise. If you don't need the extended functionality provided by CONVERT, use CAST.

Performance

Generally, there is no significant performance difference between CAST and CONVERT. However, implicit conversions (without using CAST or CONVERT) may result in a loss of precision.

Other considerations

• ANSI Compatibility: CAST strictly adheres to the ANSI-SQL standard, which is very beneficial for portability.
• Extended features: CONVERT provides additional options for formatting and specific conversion rules.

Example

To convert a string to a decimal number you can use:

<code class="language-sql">DECLARE @string VARCHAR(10) = '123.45';

-- 使用 CAST 进行转换：
DECLARE @decimal1 DECIMAL(10, 2) = CAST(@string AS DECIMAL);

-- 使用 CONVERT 进行转换：
DECLARE @decimal2 DECIMAL(10, 2) = CONVERT(DECIMAL(10, 2), @string);</code>

Conclusion

When choosing between CAST and CONVERT, consider the following guidelines:

• Use CAST when ANSI-SQL compatibility and simplicity are required.
• Use CONVERT when extended formatting options or specific conversion rules are required.

However, always be aware of the potential loss of precision when using implicit conversions, and use CAST whenever possible to avoid such problems.

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