Is it necessary to have Primary keys?

Could anyone tell me if it is necessary to have Primary Keys defined on all tables in your database. Are there any disadvantages to not having primary keys defined? This particular database is used only to run large batch transactions against for gathering statistics, etcs. I will have duplicate records in my table ( except perhaps for the date/time). I don't believe it is wise to put a date in a primary key. I'm a little concerned because I have always heard that it is necessary for each table to have a primary key. Can anyone shed some light on this for me??

Thanks in advance

Lisa Received on Wed Jun 04 1997 - 00:00:00 CDT