On Jan 2, 2010, at 9:04 AM, Ricardo Aráoz wrote:
>> I've worked for many clients who made claims like "we will never need
>> more than one phone number per customer" or "part numbers will never be more
>> than 4 places"
>
> That's why I'd never use the part number as a PK and use an auto
> incremental instead. When my customer wants to use more than 4 places
> and allow alphabetic characters in the part number (note I said "when"
> and not "if" ;.) ) it will be easy enough to make the necessary changes.
That was the exact situation I had with a customer, but it was an
office supply company. I took over the system that used part numbers as PKs,
and pointed out the problem, offering to switch to a surrogate key design. The
client thought I was trying to create work to pad my hours, and confidently
assured me that the 4-character part number would be all they would need.
I registered my concern but said I would respect his decision. I then
spent the next few months building their system, adding all these features to
enable their business to grow. Well, it did in fact grow, because within that
first year, they bought out a competing company. Of course, when we tried to
merge their operations together, the other company used 6-character part
numbers. Had we switched to surrogate keys at the beginning, it would have been
trivial to change a single column and a couple of display fields to fit. Now we
had lots of SQL and other code that assumed equality that would be broken once
we switched to 6 chars, since 'abcd' != 'abcde '.
It turned out to be several months worth of work to combine the
systems, and instead of saying "I told you so", I merely mentioned to the
client that step one of the process would be to switch to surrogate keys in the
tables. This time I got no resistance. ;-)
-- Ed Leafe
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