External sets are used to describe the data and format of external files for import and export tasks.
When used for export or import tasks, an external set describes the structure, format, and record separator of an external file. For an export task, it defines the format of the file to hold the exported data. For an import task, it also forms the definition of the temporary table that is used when the external file is being processed.
For example, if the Travel Agency wants to export personnel data in a form that can then be read by an external application such as Microsoft Word, they must define this format using an external set.
It is also possible to export data into HTML files that can be read using a browser such as Netscape or Internet Explorer. In this case you use a particular type of external sets referred to as HTML sets.
To take another example, when the Travel Agency wants to add new data to their database from a file created on another system, they must use an external set to define the format of the external file to be imported.
The Import SQL statements used by an import task manipulate the data in the external set, and determine which data is to go into which table. These Import SQL statements may insert new data, or update existing data.
For import tasks, it is also possible to include virtual elements in an external set. These allow you to carry out some data manipulation on the data, before it is imported into the database.
For example, when importing complex data into several tables at a time, you can improve performance by carrying out database retrievals or calculations only once, and storing these values in virtual fields of the import record.