>>And the exchange rate is a historic information, i.e. needs to be recorded with the transaction, or to be retrieved from another table with rates and dates. Keeping only the current rate in a lookup is not going to work.
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>Right, I forgot to mention that. - Actually, I was mainly thinking about the "alternative", which I often saw here in Bolivia - programmers tend to use one field for the amount in the country currency ("bolivianos"), and another field for the amount in US-dollars... which makes the system quite inflexible when it comes to adding additional currencies, such as the Euro, yen, etc., the currency of some neighboring countries, or perhaps some future Southamerican currency union. Which they may, or may not, need in the future. Currently, it is customary to use only those two currencies - the country's currency, and "foreign currency" which is slang for US-dollar.

It's usually one strong currency which gets this status of the real unit of measure, and the domestic one is the official one. Here it was the german mark back in the nineties, with the american dollar as somewhat distant second, the influence commensurate with the percentage of circulation. We had an app to track documentation of the construction of the new hospital back then, and there was a desire to recalculate all those billions and trillions into some real money. I had collected the exchange rates for the previous decade or so, and didn't really care how sparse it was - for every transaction I used the last known rate (set near on, set order to date descending, seek, go recno(0) - worked perfectly).

While RSD is rather stable last dozen years, it's still in the habit of people to speak in foreign currency at times - depending on the item. It can be quite confusing, but with the exchange rate being about 118 RSD :1 EUR, you only need to know the order of magnitude to guess which currency it is. Drywall - the guys will charge you seven (must be euros, nobody will work for 7 RSD per square meter). You may hear "paid two thousand, awful" - ah, it's 2.000 dinars for a bag of groceries, too much, and "paid two thousand, real bargain" - then it's 2.000 EUR for a used car.