Erd Model

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

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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

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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 composite e.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-valued e.g. phone number
            ▪ Attribute that holds multiple values for each occurrence of an entity type.

        o derived e.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... [continues]

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