A good database design stores each item of data in only one place, rather than having multiple copies in different files. The i OS join logical file is a mechanism for building views that assemble data from different files.
In CA 2E you use virtual fields to design such views. A virtual field is a field that is logically, but not physically, present in a file.CA 2E virtual fields are implemented through i OS join logical files.
When a file is referenced by another file by means of a Refers to relation, entries are automatically created on the referencing file for the key fields of the referenced file. These are known as foreign keys.
To include a non-key field from the referenced file in the referencing file, you need to specify it as a virtual field. This makes the field available for use in the functions that operate upon the referencing file.
For example, to include the name of the horse when displaying the RACE ENTRY file, specify Horse name as a virtual field on the RACE ENTRY file. As a result, Horse name is physically present on the HORSE file, and is logically, but not physically, present on the RACE ENTRY file. This is shown in italics in the following figure:
Note: This same capability is available when two files are associated with each other by means of an Owned by relation. In other words, non-key fields from the owning file may be included in the owned file as virtual fields.
For further details on virtual fields, see the Tutorial.
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