Please take a look at the official IBM documentation: http://www-01.ibm.com/support/knowledgecenter/SSEPGG_10.5.0/com.ibm.db2.luw.sql.ref.doc/doc/r0059211.html?cp=SSEPGG_10.5.0%2F2-12-6-1-5&lang=de

We are not changing what the user is requesting, it just doesn't render the null precedence which the database would use anyways. The DB2 grammar simply does not allow you to make it explicit.