SQL & PL/SQL

Hello,

I have a table, table1 that has 10 columns of which one is a serial number and another is DOB. For some rows in the table there are duplicate serial numbers. There should be no serial number duplicates.
for example:

123456 11/01/75
123456 NULL

so when do a select distinct it shows both the records because of the missing DOB.

How can I delete the record that does not have the DOB?

Another issue with this table is that it holds history information of clients. So for any given client I have 4 diffrent address information rows. I need to delete the 3 old rows and retain only the latest address info. This data is also in another table table2 that has the date that there was the address change. So I will need to pick up the latest address from table2 and delete the rest from table1.

How can I delete the old address information?

any help is very much appreciated.
Thank you.