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Erd Model

By | Feb. 2013
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CHAPTER 11: ENTITY-RELATIONSHIP MODELLING

What is it about?

• ER model is used to show the Conceptual schema of an organisation.

• Independent of specific data model or DBMS

• The model is later transformed into a Logical model (e.g. relational) on which the physical database is built

• The most widely used form of Semantic modelling: attempt to capitalise on knowledge of meaning of data to inform the model

Perspective of the Entity Relationship Model

[pic]

Concepts of the ER Model

– The basics of Entity-Relationship modelling

– Entities

– Relationships

– Attributes

Entity Type

• Entity - distinguishable “thing” in the real world

– Strong (or regular) entity - entities have an independent existence (e.g. staff)

– Weak entity - existence dependent on some other entity (e.g. next of kin)

ER diagram of Staff and Branch entity types

[pic]

Attributes

• Entity types have Attributes (or properties) which associate each entity with a value from a domain of values for that attribute

• Attributes can be

o simple (atomic)e.g. Surname; date of birth
▪ Attribute composed of a single component with an independent existence

o compositee.g. address (street, town, postcode)
▪ Attribute composed of multiple components, each with an independent existence. o single-valued
▪ Attribute that holds a single value for each occurrence of an entity type.

o multi-valuede.g. phone number
▪ Attribute that holds multiple values for each occurrence of an entity type.

o derivede.g. D.O.B. ; age
▪ Attribute that represents a value that is derivable from value of a related attribute, or set of attributes, not necessarily in the same entity type.

o Relationship types can also have attributes! (see...

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