ER Diagrams 判断题

ER Diagrams [18 points]

[4 points] True or False:

1. ER diagrams are used to model the physical schema of a database.

    False, they are used to model the logical schema. Physical schema refers to things like file and index layout.

2. An entity set E can be associated with a relationship set R more than once.

    True, for example a relationship Reports_To which has the entity Employees participating twice, since Managers are also Employees.

3. At most 2 distinct entity sets can be associated with any relationship set R.

    False, for example, a ternary relationship “Covers” with participating entities Employees, Dependents, and Policies.

4. A weak entity set must have total participation in its identifying relationship set.

    True, by definition.