Question

Is it possible to create a udf where the return type is the same as the first argument.

Other notes:

• The first argument can be any type and if it is null the return value is null with no underlying type.
• Constraints on the second argument so the type matches that of the first.

http://msdn.microsoft.com/en-us/library/aa933210(v=SQL.80).aspx

Answer 1

You could try using the sql_variant datatype for a UDF. (I’ve haven’t BTW)

However, a simple in-line ISNULL use the datatype of the 1st argument anyway.

Unless you haven’t given us full information in the question about what you intend to achieve…

Edit: after update

The whole point of ISNULL is to replace a NULL (1st argument) with a value (2nd argument).

There is no “type” of NULL. An int or a varchar can take a NULL value. But it’s still the base datatype

ISNULL constrains 2nd argument to the 1st type anyway: the 2nd argument must be implicitly convertible. So SELECT ISNULL(CAST(NULL AS int), 'foo') will fail

So, use in-line ISNULL which does what you want. A udf is not needed

Edit, after comment

You can’t have one udf to cover all datatypes.

Not least, a udf has a single return datatype.