The customers table in this sample application contains a column named balance. I've made the assumption that the values in the balance column are expressed in local currency (that is, U.S. dollars in the U.S., British pounds in the U.K.). This assumption serves us well until our corner video store opens a web site and starts accepting orders from foreign customers.
PostgreSQL doesn't have a predefined data type that represents a foreign currency value, so let's create one. You want to store three pieces of information for each foreign currency value: the name of the currency (pounds, dollars, drachma, and so on), the number of units, and the exchange rate at the time the foreign currency value was created. Call your new data type FCUR (Foreign Currency). After you have fully defined the FCUR data type, you can create tables with FCUR columns, enter and display FCUR values, convert between FCUR values and other numeric types, and use a few operators (+,-,*,/) to manipulate FCUR values.