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

Comparison of CAST and CONVERT in T-SQL: When to use which function?

When using T-SQL, developers often face a question: whether to use CAST or CONVERT to perform data type conversion. Although both functions basically achieve the same result, there are subtle differences between them.

Performance

Choosing one over the other will not create significant performance issues. Both CAST and CONVERT are implemented efficiently in SQL Server. However, it is worth noting that implicit conversions (i.e. not using CAST or CONVERT) may result in a loss of precision.

ANSI Compatibility

CAST is ANSI-SQL standard, while CONVERT is SQL Server specific. This means CAST has wider support across different database platforms.

General guidance

Based on the above factors, here are general guidelines for when to use each function:

• Use CAST:

  • When you need to perform simple type conversion, such as from integer to string or vice versa.
  • When you need ANSI-compliant code to run on different platforms.

• Use CONVERT:

  • When you need to perform custom format conversion, such as applying a specific date format.
  • When you need to convert between data types that CAST does not directly support.

It is generally recommended to use CAST whenever possible because it is standardized and as efficient as CONVERT. However, if you need CONVERT's extended capabilities, such as custom formatting, then it is a more suitable choice.

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