dd and mbr

The command:

dd if=/dev/zero of=/dev/da0

does not delete the mbr, I still see the partition table with
fdisk. My questions:

(1) What does this command do?

(2) How to delete the mbr with dd? How to address the whole disk with dd?

In OpenBSD is simply

dd if=/dev/zero of=/dev/rsd0c

with perhaps a count argument.

Rod.