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is it advisable for me to merge them into the same table?

It depends on how you're going to use the data.

If yes, How do i verify whether they are visitors or occupants?

It all depends on how you are distinguishing between occupant and visitor. If you mean, "How do I distinguish them in the database?" then you can add another field to differentiate them.

1 more question. if i add another field into the table to differentiate them, should that field be my primary key or other fields?

It can't be a primary key, since by definition primary key values are unique.

I would, however, most likely index that field.

thanks for your help =)