Beginning in SAS 9 of PROC SQL, a warning is written to the SAS log when
creating a table with the same name as any table listed on the FROM
clause.

The '92 ANSI standards for SQL state that recursive table references are
illegal unless the SQL conforms to the "FULL SQL" ANSI standard or the
implementation chooses to allow recursive references to be used.  PROC
SQL does not follow the "FULL SQL" ANSI standard.  Recursive references,
used in update operations, can return incorrect results.

It is for these reasons recursive references are not supported.

The "recursive reference" warning can be prevented for the CREATE TABLE
statement in SAS 9 by including the UNDO_POLICY=NONE option on the PROC
SQL statement.  The use of the UNDO_POLICY=NONE option will only prevent
the warning for SQL queries similar to the following: 

   proc sql undo_policy=none;
     create table one as 
     select * from one; 
   quit;

Note: The use of the UNDO_POLICY=NONE option only prevents the warning.
It does not remove the inherent risk possible if recursive table
references are chosen to be used.

The UNDO_POLICY=NONE option will not prevent warnings if recursive references
are being made in UPDATE / INSERT / DELETE operations.  Similar to:

   proc sql undo_policy=none;
     delete from one 
     where id in (select id from one); 
   quit; 

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