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Web Technologies-II

Notes Figure 9.3: Relational Data Model

SALES CUSTOMER
TERRITORY

SALES
PERSON ORDER PAYMENT

ORDER LINE
ITEM

Relationship links NOT through physical
pointers, but by foreign keys

9.4.3 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 customer in the foreign
key field.
9.4.4 No Physical Pointer

Unlike the hierarchical or the network data models, the relational model establishes relationships
between data structures by means of foreign keys and not by physical pointers.

Dr. E. F. Codd is the father of the relational model, stipulated the rules and
put this model on a solid mathematical foundation.

Self Assessment Questions
1. There are two aspects of the data about customers.
(a) True (b) False
2. Oracle and Informix are not really the software that manages data.
(a) True (b) False
3. The data repository contains the actual data.
(a) True (b) False
4. …………………….. occurrences of an owner record type with the corresponding occurrences
of the member record type.
(a) ListItem element (b) Markup Language
(c) Encapsulation (d) Physical pointers link
5. Physical pointers link records of the parent segments to those of the child segments by
means of parent-child forward or ………………………..
(a) NET provides (b) components
(c) Backward pointers (d) Compatibility

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