Basic Elements of E-R Model Entity Object of the real world
that stores data. Eg. Customer, State, Project, Supplier, etc.
Attribute A description of an entity. Eg. Worker has name, worker
number, department, address, etc. Relationship
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Example of an Attribute
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Type of Attribute Simple Attributes vs. Composite
Attributes
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Solitary Attribute vs. Multiple Value Attribute
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Derived Attribute
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Attribute Domain & Key Set of values for an attribute
Character Numeric Date Attributes with one key/ two keys
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Relationship Link between entities. Types of relationship:
Unary Binary Ternary
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Unary
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Binary
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Ternary
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Cardinality One to one relationship (1:1) One to many
relationship (1:M) Many to many relationship (M:N)
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1:1 Relationship
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1:M Relationship
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M:N Relationship
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Relationship Participation Compulsory Optional
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Guidelines and Steps in ER Model No system environment
insertion.
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Wrap-up on ERD Determine entities, relationship. Determine
attributes. Determine attributes related to relationship (if any).
Choose the keys for the entities. Determine the domain for each
attribute. Combine the diagrams. Check and refine.
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Tutorial 1 Prepare an ERD for the following case study: A
university database contains information about lecturers
(identified by staff number) and courses (identified by course
code). Lecturers teach courses; each of the following situations
concerns the Teaches relationship set. For each situation, create
an Entity Relationship (ER) diagram that describes it (assuming no
further constraints hold). 1. Lecturers can teach the same course
in several semesters, and each course offering must be recorded. 2.
Every lecturer must teach some course. 3. Every lecturer teaches
exactly one course (no more, no less). 4. Every lecturer teaches
exactly one course (no more, no less), and every course must be
taught by some lecturer. 5. Now suppose that certain courses can be
taught by a team of lecturers jointly, but it is possible that no
one lecturer in a team can teach the course. Model this situation,
introducing additional entity sets and relationship sets if
necessary.