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The basics of the entity-relationship model in database systems, including entity sets, attributes, relationship sets, mapping constraints, keys, and e-r diagrams. It also discusses weak entity sets, generalization, and specialization.
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(^) Entity Sets (^) Relationship Sets (^) Mapping Constraints (^) Keys (^) E-R Diagram (^) Weak Entity Sets (^) Generalization & Specialization (^) Exercise on E-R Diagrams
(^) A database can be modeled as: (^) a collection of entities, (^) relationship among entities. (^) An entity is an object that exists and is distinguishable from other objects.
(^) Entities have attributes (^) Example: people have names and addresses (^) An entity set is a set of entities of the same type that share the same properties. (^) Example: set of all persons, companies, trees, holidays
(^) An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set. (^) Domain – the set of permitted values for each attribute (^) Attribute types: (^) Simple and composite attributes. (^) Single-valued and multi-valued attributes (^) E.g. multi-valued attribute: phone-numbers, mobile numbers, emails etc. (^) Derived attributes (^) Can be computed from other attributes (^) E.g. age , given date of birth Example: customer = (customer-id, customer-name, customer-street, customer-city) loan = (loan-number, amount)
Composite Attributes Composite Attributes
(^) An attribute can also be property of a relationship set. (^) For instance, the depositor relationship set between entity sets customer and account may have the attribute access-date
(^) Express the number of entities to which another entity can be associated via a relationship set. (^) Most useful in describing binary relationship sets. (^) For a binary relationship set the mapping cardinality must be one of the following types: (^) One to one (^) One to many (^) Many to one (^) Many to many
One to one One to many Note: Some elements in A and B may not be mapped to any elements in the other set
Mapping Cardinalities affect ER Design Mapping Cardinalities affect ER Design (^) Can make access-date an attribute of account, instead of a relationship attribute, if each account can have only one customer (^) I.e., the relationship from account to customer is many to one, or equivalently, customer to account is one to many
(^) Rectangles represent entity sets. (^) Diamonds represent relationship sets. (^) Lines link attributes to entity sets and entity sets to relationship sets. (^) Ellipses represent attributes (^) Double ellipses represent multivalued attributes. (^) Dashed ellipses denote derived attributes. (^) Underline indicates primary key attributes (will study later)
(^) Entity sets of a relationship need not be distinct (^) The labels “manager” and “worker” are called roles; they specify how employee entities interact via the works-for relationship set. (^) Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. (^) Role labels are optional, and are used to clarify semantics of the relationship
(^) In the one-to-many relationship a loan is associated with at most one customer via borrower , a customer is associated with several loans via borrower
(^) In a many-to-one relationship a loan is associated with several customers via borrower , a customer is associated with at most one loan via borrower
