Mcdonnell douglas aircraft design database scheme

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Unformatted text preview: ubishi radio : string space : real cargoaircraft floor : real B100.kitchens:intg wheelsys:dunlop3 strategicbomber MD10A weapontrans : GE refuelsys:shell commsys:siemens radar : comsat range : real country:string range : integer cockpit:bombardier B100 bomberunit : string computer : cray airforce : string 12 object attribute connection attribute/ method connection kitchen hierarchy or subclass association one-many relationship connection standard : string Legend tseat : integer bombs : {string} tseat(classcat):integer weapon : {string} missileunit : integer MD10.kitchens:intgr The characterizing properties of abstract objects are called attributes. As opposed to the object-oriented practice, properties are grouped around objects and only semantically relevant properties are attached to an object as is the case in relational model. Objects store data in attributes in order to save their states, and respond to instructions for carrying out speci c operations on their attributes when a request for an operation, a message, is received by an object. A message speci cation always includes a context (i.e. a recipient) saying which object should receive and respond to a message. The set of attributes that can be accessed or the set of operations that can be performed on an object are determined by the object. Thus an object exhibits its behavior in terms of these attributes and operations. In the example of Figure 1, aircraft, DC10-30, seat, etc. represent (classes of) objects. The physical identity of an object is hidden from the user if the object is a semantic object. In case of entities, a set of atomic attributes uniquely identi es an object in its class { a key and is thus available to users. Semantic objects in OR model exhibit a duality | as a class and as a simple object, called an instance. Similar kinds of objects may be grouped into classes of semantic objects or entity sets of entities. Unlike other SDMs (e.g., 36]), we do not distinguish between attributes that describe properties of class objects or member objects. We will take up this two issues again later in this section. 2.3.2 Types of Objects Similar to other semantic data models, OR model also has the provision for direct representation of object types distinct from their attributes and sub- or supertypes. Concrete objects in OR model are said to have basic types and abstract objects have constructed types. They both correspond to the atomic types in SDMs. However, we do not consider abstract types to be atomic, rather we view them as complex2 since they may have arbitrary nested structures. Our model supports aggregation type 39], and grouping (or association) type 39] constructors to create new types of objects from the basic and constructed ones. An aggregation is a composite object constructed from other objects in the database. For example, each object associated with the aggregation type useseat in Figure 1, is an ordered triple of aircraft, seat and In this thesis we do not make a distinction between complex objects and composite objects, and thus use these terms interchangeably. However, Kim et al. 48, 49] makes a clear distinction between them by bringing in is-part-of relationship between objects { a construct that we do not incorporate. 2 13 classcategory type objects. Mathematically, an aggregation is an ordered n;tuple. An instance of an aggregation type will be a subset of the Cartesian product of the active domains assigned to the participating nodes in it. Note that the identity of an aggregation object depends on the component values of the object. Also note that in the sense of ER model, entities can be viewed as an aggregation of basic types, while a relationship is an aggregation of at least two or more constructed types. We also take the same view and treat relationship types to be di erent from the entity types and say that all entities are persistent objects while the objects in relationships are not. It is easy to see that aggregation types (entities and relationships) in OR model are value based. We denote semantic object types and entity types using a rectangle in the OR model whereas the relationships are denoted by diamonds. The grouping constructs allow us to model sets of objects of the same type. Mathematically speaking, a grouping is a nite set. An instance of a grouping type will hold a nite subset of objects of the active domain of the node of the grouping type. A grouping object always has exactly one child (facility in seat). The identity of a grouping object is determined completely by that set. Most of the type constructors of OR model may be applied recursively and numerous kinds of types can be constructed using these simple constructors. But soon we will see that OR model only allows certain kinds of types by imposing a global set of restrictions on the type of applications of the constructors. 2.3.3 Domains and Values In the OR model, there are two types of values { basic values and constructed values. Basic values are...
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This document was uploaded on 01/10/2011.

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