I am trying to make this query work:

select ARPLOG.* from ARPLOG where WHEN_ADDED > DATE_SUB(current_date(), interval 3 month) left join dhcpd on ARPLOG.MAC_ADDR=dhcpd.MAC_ADDR where dhcpd.MAC_ADDR is null;

From these working queries:

select ARPLOG.* from ARPLOG left join dhcpd on ARPLOG.MAC_ADDR=dhcpd.MAC_ADDR where dhcpd.MAC_ADDR is null;

select * from ARPLOG where WHEN_ADDED > DATE_SUB(current_date(), interval 3 month);