CAST vs. CONVERT in T-SQL: Which Data Type Conversion Method Should You Choose?

T-SQL data type conversion: Comparison of CAST and CONVERT

SQL Server provides two main data type conversion methods: CAST and CONVERT. The two have similar functionality, but understanding the key differences can help you optimize code performance and adhere to standards.

The difference between CAST and CONVERT

• ANSI standards compatibility: CAST complies with the ANSI SQL standard, while CONVERT is SQL Server specific.
• Precision: CAST preserves the precision of numeric values ​​by truncating rather than rounding. CONVERT may cause loss of precision during implicit conversions.
• Flexibility: CONVERT provides more flexibility in formatting and customizing conversions. However, for basic type changes, CAST is sufficient.

Performance Considerations

Typically, CAST performs slightly faster than CONVERT for simple transformations because it follows industry standards. CONVERT Additional processing may be required for custom formatting.

Suggestion

For most cases, it is recommended to use CAST for type conversion. It complies with the ANSI standard, preserves accuracy, and is generally faster. However, if custom formatting or advanced conversions are required, CONVERT can be used.

Example

Consider the following example:

DECLARE @value VARCHAR(50) = '1234.5'

-- 使用 CAST 转换为整数
SELECT CAST(@value AS INT)

-- 使用 CONVERT 和自定义格式转换为整数
SELECT CONVERT(INT, @value, 1)

The result of CAST will be 1234 and the result of CONVERT will be 1,234 because of the custom comma delimiter used.

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