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7.1 Mandatory Fields

Sometimes, you want to make sure that every record of a particular type contains certain fields. To do this, use the special field %mandatory. The usage is:

     %mandatory: field1 field2 ... fieldN

The field names are separated by one or more blank characters.

The fields listed in a %mandatory entry are non-optional; i.e. at least one field with this name shall be present in any record of this kind. Records violating this restriction are invalid and a checking tool will report the situation as a data integrity failure.

Consider for example an “address book” database where each record stores the information associated with a contact. The records will be heterogeneous, in the sense they won't all contain exactly the same fields: the contact of an Internet shop will probably have a URL field, while the entry for our grandmother probably won't. We still want to make sure that every entry has a field with the name of the contact. In this case, we could use %mandatory as follows:

     %rec: Contact
     %mandatory: Name
     Name: Granny
     Phone: +12 23456677
     Name: Yoyodyne Corp.
     Phone: +98 43434433

A word of caution, however: In many situations, especially in day to day social interaction, it is common to find that certain information is simply unavailable. For example, although every person has a date of birth, some people will refuse to provide that information.

It is probably wise therefore to avoid stipulating a field as mandatory, unless it is essential to the enterprise. Otherwise, a data entry clerk faced with this situation will have to make the choice between dropping the entry entirely or entering some fake data to keep the system happy.