The @ Notation - Clarity

[Len Conrad]

>zone "xyz.net" in {     type master;    file "db.xyz.net"; };

you need the trailing  "};" to enclose the sub-statements list, and to 
terminate the zone statement.

you can drop the IN class, it's the default

>And the following in my zone data file (xyz.net) :
>
>$TTL 3h

$TTL 1d

would be better.

>@ SOA host-123.xyz.net. admin.yxz.net. 2002120501 8h 1h 1w 1h

You can drop the IN class of network everywhere in zone file, it's taken 
from the zone statement.

the ccyymmddxx format for the serial number is useful

>And the hosts would still be build correctly to be, for example 
>host-123.xyz.net. as the @ notation completes the current origin, that 
>being xyz.net.

yes

Len