CELKO (4/22/2015)
An identifier (<something>_id) is measured on a nominal scale. Math makes no sense on such scales -- what is your credit card number divided by the UPC code on your candy bar?Therefore, identifiers are strings.
Credit card number is not an identifier.
When you card is expired you get a replacement one with the same number. So, you get 2 or more cards with the same number, but the cards are not identical: you cannot use them the same way.
For a credit card it's a combination of string(number) and date(expiry date) values which identify a card. And expiry date is a DATE, NOT STRING, because expressions 04/2015, 04-2015, 04/15, 04-15 present the same value.
Therefore, identifiers may be of any data type.
Whatever is suitable in any particular situation.
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Code for TallyGenerator