5.4 Independent and dependent entities

Some ER models may:

1. Allow relationships between three or more entities. 2. Decompose a complex model into a group of related entities, called a subject area. 3. Refer to independent entities as strong and dependent entities as weak.

Some ER diagrams may

1. Depict relationship names inside a diamond. 2. Depict dependency relationships with a diamond rather than an arrowhead. 3. Use color, dashed lines, or double lines to convey additional information.

Several model and diagram conventions

1. UML 2. IDEF1X 3. Chen notation

depicts cardinality as follows

1. Zero — a circle across the end of a relationship 2. One — a bar across the end of a relationship 3. Many — three lines, called crow's feet, at the end of a relationship

Distinguish independent and dependent entities.

3a. Identify independent and dependent entities. 3b. Determine the dependency relationship for each dependent entity. 3c. Document dependent entities and dependency relationshipsin glossary and ER diagram.

Dependent Entity

An entity that requires a relationship to another entity for identification.

dependency relationship

The dependent and master entities are related by a

Database designers can look for dependency relationships first, before identifying dependent entities. T or F

True- Some database designers prefer to focus on dependency relationships rather than dependent entities.

Chen notation

appeared in an early ER modeling paper by Peter Chen. Chen notation is not standardized but often appears in literature and tools.

master entity

dependent entity relies on this

independent entity

does not depend on any other entity.

entity-relationship model, dependence mean

each instance of a dependent entity always relates to an instance of another entity.

each instance of a dependent entity always relates to an instance of another entity.

each value of a column relates to at most one value of another column.

entity-relationship dependence is called

existence dependence

relational dependence is called

functional dependence

Unified Modeling Language (UML)

is commonly used for software development. Software data structures are similar to database structures, so UML includes ER conventions.

IDEF1X

stands for Information DEFinition version 1X. IDEF1X became popular, in part, due to early adoption by the United States Department of Defense.

independent

strong

group of related entities

subject area

dependent

weak