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
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
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.
e.g. phone number
▪ Attribute that holds multiple values for each occurrence of an entity type.
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 later)
• A relationship is
“... An association among entities (the participants)...”
• Relationships link entities with each other
Name: verb, capital start letter, arrow indicates
direction in which verb makes sense
← Relationship type
← Set of meaningful associations among entity types.
← Relationship occurrence
← Uniquely identifiable association, which includes one occurrence from each participating entity type.
Semantic net of Has relationship type
ER diagram of Branch Has Staff relationship
← Degree of a Relationship
← Number of participating entities in relationship.
← Relationship of degree :
← two is binary
← three is ternary
← four is quaternary.
← Recursive Relationship
← Relationship type where same entity type participates more than once in different roles.
← Relationships may be given role names to indicate purpose that each participating entity type plays in a relationship.
Recursive relationship called Supervises with role names
Entities associated through two distinct relationships with role names
← Candidate Key
← Minimal set of attributes that uniquely identifies each occurrence of an entity type. ← Primary Key
← Candidate key selected to uniquely identify each occurrence of an entity type. ← Composite Key
← A candidate key that consists of two or more attributes. ER diagram of Staff and Branch entities and their attributes
← Strong Entity Type
← Entity type that is not existence-dependent on some other entity type. ← Weak Entity Type...
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