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
Concepts of the ER Model
– The basics of Entity-Relationship modelling
• 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
• 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.
▪ 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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