SPELL supports the definition of binary relationships. The set of SPELL relation types is organized in a hierarchy, whose root is PM_ Rel. A SPELL relation type is defined as shown in Figure 1.10.
Figure 1.10: A relation type
All relationships are binary. The two connected
entity types, destination and source types, are expressed by the
Cardinalities are expressed through lower and upper bounds, where the lower bound must be either 0 or 1, and the upper bound 1 or N.
Access to Relationships is performed from the related objects by using procedures cre_ rel, read_ rel, or del_ rel. Futhermore, a relationship cannot be updated.
A relationship can be created by calling the procedure:
cre_ rel(RelationName, RelatedId,AttrList);
Thus a relationship of type RelationName is created from
$self to RelatedId, specifying the attribute values
given in AttrList.
The use of read_ rel is analogous. In order to make these concepts clearer we present some of the relations involved with tasks and products (always refer to 1.4).