Generate unique ID based on records in table

A better design would be to generate the id from a sequence. First, create a sequence similar to the following:

CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1;

Then create a trigger to automatically populate your table's column with the next number in the sequence:

CREATE TRIGGER my_tab_trig
BEFORE INSERT ON my_table
AS
   next_num NUMBER;
BEGIN
   SELECT my_sequence.NEXTVAL INTO next_num FROM dual;
   :new.my_column := next_num;
END;
/

When a user inserts a row of data into the table, Oracle will run the trigger which will set the column value to the next unique number in the sequence. Forgive me if the above code is not perfectly correct as I typed it all off the top of my head. But this is definitely enough to get you started.