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Working with user defined functions (UDF)

You SHOULD:-

  • Use inline table valued functions instead of scalar functions where possible.

  • Use WITH SCHEMABINDING for non-data accessing scalar user defined functions. It forces SQL server to check data access is occurring at design time. Thus, preventing the need to do so during execution.

You SHOULD NOT:-

  • Use multi-statement table valued functions; these can impede performance.

  • Use side-effecting operators within a user defined function, for example SELECT, PRINT, INSERT, UPDATE, TRY, CATCH

Further reading and information

TSQL User-Defined Functions: Ten Questions You Were Too Shy to Ask - Simple Talk (red-gate.com)