Use parameter markers or the NULL value as function arguments

An important restriction involves both parameter markers and the NULL value.

You cannot code the following: 

     BLOOP(?)

or 

     BLOOP(NULL)

Since function resolution does not know what data type the argument may turn out to be, it cannot resolve the reference. You can use the CAST specification to provide a data type for the parameter marker or NULL value that function resolution can use:

     BLOOP(CAST(? AS INTEGER))

or 

     BLOOP(CAST(NULL AS INTEGER))
Parent topic: Use UDFs in SQL statements