Aiko,

The use of a surrogate key has many advantages. One fo the biggest is that there is no need to deal with changes to the value of the key. With a "natural" key, the users may want to change its value and that requires that all foreign keys that refer to the PK must be changed as well. The user will never desire to change what they can't see, so with surrogates there is no need to cascade changes.

Most folks use a siple sequential number for the key. Since they are meaningless, there is no reason to be concerned with missing numbers. I use Integer fields for PK's since they provide for over 2,000,000,000 positive values in 4 bytes. I keep a key table with one record for each table in the system and just increment the value in the table for each append that occurs.