Key Rule

This does not require that a column be defined for all of the data items in every key. Nor does it preclude part of the key from using default values, or part of the key being defined in the filter. It is also allowable to have no parts of the key defined by columns. This could happen if you have a key that used spaces as a valid value for some form of counter or header record. This would allow only one insert, but would be valid for updating. Some front-end tools may have different requirements. As an example, Microsoft Access wants to be able to determine a unique index before it will allow updates or inserts in datasheet view.

In this example, the data items, the following highlighted items make up the primary key. All of these data items should be accounted for in any tables that will allow write operations. They may be accounted for as a column, as part of the record type filter, or as part of the table filter.