Not all information necessary for business purposes may be stored in the same place. Even if the data resides in locations within the firewalls of the business, it may be in different applications or formats. In the example below from an Oracle administrator’s manual, data stored in different, distributed repositories can be a good thing, but there are many variables to consider.

This is an ideal example of a well-architected database system where users are getting maximum value from three separate systems that seamlessly working together as one. In most cases, external data—or data not residing in the current database—is much trickier to utilize. External data is usually stored in differing applications in differing formats. Access can use data in the following formats: Oracle, SQL Server and other ODBC compliant databases, Excel, XML, HTML, Word documents and text files.