[feladat @ 240] Bugfix: when a zone has no records (eg a incorrect slave) it was not possible to delete this zone. Modified the query to fetch the records of the zone with a LEFT OUTER JOIN so there still will be a result when an empty zone is selected. This closes ticket:25. Bugfix: Made the query compatible with pgsql

See <https://code.krikkit.nl/trac/poweradmin/wiki/ReleaseNotes/2.0.0>.