*  The ON clause cannot include any subqueries.
*  Column references in an ON clause must only reference columns of
   tables that are in the scope of the ON clause.
*  Scalar fullselects are not allowed in the expressions of an ON
   clause.
*  A function referenced in an ON clause of a full outer join must be
   deterministic and have no external action.
*  A dereference operation (->) cannot be used.
 *  A SQL function or SQL method cannot be used. 
*  The ON clause cannot include an XMLQUERY or XMLEXISTS expression.