Copy data from one table to another, where some records might already be present

Hello !

I have two tables with identical schema. I want to copy all data from
Table A to Table B, but table B might already have some of table A:s
data (each record is identified using record_numbers). I would suspect
this can be accomlished using a 2 stage query, first performing a join
of some kind and then the copying. But I have little to no clue on how
to make it happen.

Any ideas ?

Thanks a lot !

/Otto Blomqvist