Row based keys or composite key

hi

few question regarding row based keys ( user-generated or system generated).

1. if we define a composite primary key with several fields, can these fields allow null values in them?
2. Is there a simple way to generate a UNIQUE number for each distinct row? ( say if there are three fields f1,f2,f3 in a table, rows are distinct if f1||f2||f3 are different in each row) i would like to get a unique number corr. to each distinct row, like row1 num1 row2 num2 row3 num3 where num1 != num2 != num3

Thanks in advance for your suggestions. --
Nandakumar

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