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Databases

Entity-relationship modelling

  • 3 things to know
  • A2 Unit 1

Overview

Entity-relationship model, one-to-one relationship and one-to-many relationship.

What you need to know

Answer each one before you reveal it.

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Key term

Why can a many-to-many relationship not be implemented directly?

Key term

What is meant by the optionality of a relationship?

Key term

Produce the entity-relationship model for a library that lends copies of books to members.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is an entity-relationship model?

    Model answer

    An entity-relationship model is a diagram showing the entities a system must hold data about, the relationships between them, and the degree of each relationship. It is the standard way of agreeing the logical structure of a database before it is built.

  2. 2

    What are the three degrees of relationship shown in an entity-relationship model?

    Model answer

    The three degrees are one-to-one, one-to-many and many-to-many, describing how many occurrences of each entity may be associated with an occurrence of the other.

  3. 3

    Give an example of a one-to-one relationship.

    Model answer

    An employee has one company car and a company car is allocated to one employee, so the relationship between Employee and Car is one-to-one.

  4. 4

    Give an example of a one-to-many relationship.

    Model answer

    One customer may place many orders but each order is placed by exactly one customer, so the relationship between Customer and Order is one-to-many.

  5. 5

    Give an example of a many-to-many relationship.

    Model answer

    A student studies many courses and a course is studied by many students, so the relationship between Student and Course is many-to-many.

  6. 6

    How is a one-to-many relationship implemented in tables?

    Model answer

    The primary key of the table on the one side is placed into the table on the many side as a foreign key, so each order carries the customer number of the customer who placed it.

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