Page 249 - DCAP405_SOFTWARE_ENGINEERING

Software Engineering

Notes The objective of the E-R designing activity is to obtain these components of the E-R model.
E-R diagrams are very useful tools to develop and describe the E-R model of a database. In its
original form ER diagrams were proposed by Dr. Pin-Shan Chen in the year 1976. The popularity
of the tools caused many variants come up since then.
E-R diagrams are composed of symbolic notations for entities, attributes and relationships
along with other aspects of these. A few of the symbolic notations and their meaning are listed
in the Table 14.1.

Table 14.1: Symbols and their meaning used in ER-diagrams

Symbol Meaning

Entity

Single-valued
attribute

Relationship

Connector

Derived attribute

Multi-valued
attribute

Existence
dependency

Identifying
relationship

Did u know? What are the basic elements of ER Models?
There are three basic elements in ER models:

1. Entities are the “things” about which we seek information.
2. Attributes are the data we collect about the entities.
3. Relationships provide the structure needed to draw information from multiple
entities.
This list is not complete. Other symbols will be covered later. Using these symbols one can draw
an entity-relationship diagram for a database. ER diagram for the entity AccountHolder is
shown in figure 14.5(a) and for entity Account is shown in figure 14.5(b).

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