Re: Database theory and money

Alan wrote:

> In that case, use two columns, one for the amount, and one for the currency
> type.
>
> "Chad" <cthomas500_at_home.com> wrote in message
> news:0D3v5.29946$E_6.11742167_at_news3.rdc1.on.home.com...
> > Thanks for the feedback. I am familiar with Date's book. This just happens
 

> > to be the first system where I have had to deal with multiple currencies.
> > I'm looking for a good reference on this topic.
> >
> > For the record, I would allways store the currency indicator, but never in
> > the same column. This particular system does not need to compare values in
> > this column against other values in this column.
> >
> > Chad Thomas
> >
> > "Jan Hidders" <hidders_at_REMOVE.THIS.win.tue.nl> wrote in message
> > news:8pibtr$qf4$1_at_news.tue.nl...
> > > Chad wrote:
> > > > I just want some feedback on storing monetary values of different
> > > > currencies in the same column in a table. I think it violates
 relational
> > > > principles.
> > > >
> > > > Does anyone have a good source on this?
> > >
> > > Look at "An Introduction to Database Systems" by C.J. Date. One principle
 is
> > > that every column contains only values from a certain domain. Such a
 domain
> > > is a set of (atomic) values with a specific meaning. So the domains
 'weight'
> > > and 'height' may contain the same values but still are different domains.
 An
> > > important property of values within a domain is that it must make sense to
 

> > > ask if they are equal or not. For example, it usually does not make sense
> > > to ask if weight X is equal to height Y, so these should be different
> > > domains and therefore never in the same column.
> > >
> > > Whether this is the case in your table depends on what you exactly store
 in
> > > the column. If it is just a number and the monetary unit is not indicated
> > > anywhere then you probably have a problem anyway. If the monetary unit is
> > > indicated somehow in another column then this may or may not violate the
> > > principle mentioned depending upon whether you think it makes sense to
> > > compare numbers of liras to numbers of dollars et cetera. (I think it
 does,
> > > so as far as I am concerned you are not violating the principle.) If the
> > > monetary unit is indicated in the same column then you are violating the
> > > principle that values in columns should be atomic, i.e., cannot be split
> > > into meaningful parts. But this latter principle is usually not considered
 

> > > very important anyway (ususally a date is represented in one column
 although
> > > by this principle it should be split).

In my experience, there are three (or four) values of interest as soon as you are dealing with more than one currency. They are the numeric amount, the currency, and the date. Most of the time, you are going to need to convert between different currencies, and since exchange rates fluctuate, the date is also important. Further (and this is the 'or four' bit), there can be more than one exchange rate between two given currencies on a given date. Even ignoring the intricacies of currency trading on the foreign exchanges (basically analogous to stock market trading), there can be the closing rate on a given exchange on a given day, or a weekly or monthly average of some sort, or a futures contract exchange rate, or the rate bought as a future at some time in the past, or ...

Ideally, you would be using a DBMS that allows you to build your own data types, and you'd build a CURRENCY type which includes (at minimum) the amount and the currency. For your current stated purposes, that would be sufficient (dates do not appear to be relevant at the moment; I suspect that will change, but I could easily be wrong). If you are working in a system which does not allow you to build your own data types, then you can simulate it with two columns, amount and currency, as suggested.

--
Jonathan Leffler (jleffler_at_informix.com, jleffler_at_earthlink.net)
Guardian of DBD::Informix v1.00.PC1 -- see http://www.perl.com/CPAN
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