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CLASSs can have either single-valued or multi-valued properties. Graphically, single-valued properties are indicated with single-headed arrows, while multi-valued properties are indicated with double-headed arrows. In the meta schema, single-valued and multivalued properties are distinguished using the sv/mv boolean-valued attribute of the PROPERTY class (i.e., true ff the property is single-valued and false otherwise). A PROPERTY is viewed as a function that maps objects in the domain to objects in the range. PROP ERTYs can be inverses of other PROPERTYs, as indicated by the inverse-of attribute of the PROPERTY class. PROPERTYs can also be characterized as required (no null values) or unique (1:1). Graphically, required property names S I G M O D R E C O R D , Vol . 20, No. 4, D e c e m b e r 1991 55 are indicated with an "(R)" following the property name. An ABSTRACT-CLASS can also have composite KEYs which are formed from one or more PROPERTY objects. Finally, DISJOINT-CATEGORYs are a means of specifying disjoint subclasses for a given superclass. The superclass is the category-owner while the subclasses are the category-members. Disjoint classes are denoted graphically with an arc through the appropriate subclasses of the superclass. In addition to using the semantic model for representing its own structural semantics, the model is also used to describe the semantics of the data models employed by the local schemas that map to the global schema. The bottom portion of Figure 1 illustrates the use of the semantic model to describe the structure of a relational schema. The description is similar to the self-describing view of the relational model as presented by Mark and Roussopoulos in [Mark86]. The description in Figure 1 is different, however, in that the relational model is described using semantic modeling concepts. The semantic model used in Figure 1 is therefore self-describing and model-describing; in the top portion of Figure, 1 the semantic model describes itself while in the bottom portion of the figure the semantic model describes another data model. Figure 1 indicates that each RELATION is associated with a specific database and can have many ATTRIBUTEs. Each A'VI'RIBUTE has a name and a domain. ATI'RIBUTEs can also be either required and/or unique. Dependency is a boolean attribute indicating whether the A'I'I'RIBUTE object is a functional or multi-valued dependency with respect to the key of the relation. Finally, the KEY class indicates the candidate and foreign keys of each relation. The key-part attribute indicates the attributes that compose the key. Thefk-to-pk attribute links the attribute as a foreign key to its associated primary key. A similar representation of the network model is described in [Urba91]. 3 Representation of Semantic Hetero.

Resolving semantic heterogeneity through the explicit representation of data model semantics