If a stored procedure meets the following criteria, it is a good candidate for being rewritten as a table-valued function:

• The logic is expressible in a single SELECT statement but is a stored procedure, rather than a view, only because of the need for parameters. This scenario can be handled with an inline table-valued function.
  
• The stored procedure does not perform update operations, except to table variables.
  
• There is no need for dynamic EXECUTE statements
  
• The stored procedure returns one result set.
  
• The primary purpose of the stored procedure is to build intermediate results that are to be loaded into a temporary table, which is then queried in a SELECT statement. INSERT...EXEC statements can be written using table-valued functions. For example, consider the following sequence:
  
  INSERT #temp EXEC sp_getresults
  SELECT ...
  FROM #temp, t1
  WHERE ...

  The sp_getresults stored procedure can be rewritten as a table-valued function, for example fn_results(), which means the preceding statements can be rewritten as:
  
  INSERT #temp
  SELECT ...
  FROM fn_results(), t1
  WHERE ...

CLR functions provide a more reliable and scalable alternative to extended stored procedures. Many extended stored procedures perform some computational task that is harder to express in Transact-SQL. Such stored procedures can be rewritten using the CLR given the benefits described above. Further, extended stored procedures that return result sets by accessing an external resource such as a file or Web Service can be rewritten using a CLR table-valued function. For more information, see Creating CLR Functions.

Concepts

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