Database Reference
In-Depth Information
SALES
TERRITORY
CUSTOMER
SALES
PERSON
ORDER
PAYMENT
ORDER LINE
ITEM
Relationship links NOT through physical
pointers, but by foreign keys
Figure 1-10
Relational data model.
Relationships.
Consider the relationship between CUSTOMER and ORDER. For
each customer one or more orders may exist. So this customer occurrence must be
connected to all the related order occurrences. In the relational model, physical
pointers do not establish these connections. Instead, a foreign key field is included
in the ORDER data structure. In each of the order occurrences relating to a certain
customer, the foreign key contains the identification of that customer. When you
look for all the orders for a particular customer, you search through the foreign key
field of ORDER and find those order occurrences with identification of that cus-
tomer in the foreign key field. We will deal with this topic in more detail in later
chapters.
No Physical Pointers.
Unlike the hierarchical or the network data models, the rela-
tional model establishes relationships between data structures by means of foreign
keys and not by physical pointers.
Object-Relational
Take the case of the State of California Department of Water Resources (DWR),
which manages the waterways, canals, and water projects in that state. DWR main-
tains a library of more than half a million pictures. Users access this library several
times a day. A user requests a picture by content: “Show me Lake Cachuma” (a
Santa Barbara County reservoir with a low water level). Despite an index system
of captions and keywords, retrieval of the right picture within a reasonable time is
virtually impossible with the current relational database systems. Nor are purely
object-oriented data systems totally adequate to handle the challenge. As the
demand for information continues to grow, organizations need database systems
that allow representation of complex data types, user-defined sophisticated func-
tions, and user-defined operators for data access.
Object-relational database management systems (ORDBMS) present viable
solutions for handling complex data types. The object-relational model combines
the ability of object technology to handle advanced types of relationships with fea-
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