Other Styles of E-R Diagram

Posted By on September 30, 2014

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The Entity Relationship Diagram
Representation of Strong Entity Sets

Other Styles of E-R Diagram

The text uses one particular style of diagram. Many variations exist.

Some of the variations you will see are:

  • Diamonds being omitted – a link between entities indicates a relationship.
    • Less symbols, clearer picture.
    • What happens with descriptive attributes?
    • In this case, we have to create an intersection entity to possess the attributes.
  • Numbers instead of arrowheads indicating cardinality.
    • Symbols, 1, n and m used.
    • E.g. 1 to 1, 1 to n, n to m.
    • Easier to understand than arrowheads.
  • A range of numbers indicating optionality of relationship. (See Elmasri & Navathe, p 58.)
    • E.g (0,1) indicates minimum zero (optional), maximum 1.
    • Can also use (0,n), (1,1) or (1,n).
    • Typically used on near end of link – confusing at first, but gives more information.
    • E.g. entity 1 (0,1) — (1,n) entity 2 indicates that entity 1 is related to between 0 and 1 occurrences of entity 2 (optional).
    • Entity 2 is related to at least 1 and possibly many occurrences of entity 1 (mandatory).
  • Multivalued attributes may be indicated in some manner.
    • Means attribute can have more than one value.
    • E.g. hobbies.
    • Has to be normalized later on.
  • Extended E-R diagrams allowing more details/constraints in the real world to be recorded.
    • Composite attributes.
    • Derived attributes.
    • Subclasses and superclasses.
    • Generalization and specialization.

Roles in E-R Diagrams

The function that an entity plays in a relationship is called its role. Roles are normally explicit and not specified.

They are useful when the meaning of a relationship set needs clarification.

For example, the entity sets of a relationship may not be distinct. The relationship works-for might be ordered pairs of employees (first is manager, second is worker).

In the E-R diagram, this can be shown by labelling the lines connecting entities (rectangles) to relationships (diamonds). (See figure 1).

Figure 1: E-R diagram with role indicators

Weak Entity Sets in E-R Diagrams

A weak entity set is indicated by a doubly-outlined box. For example, the previously-mentioned weak entity set transaction is dependent on the strong entity set account via the relationship set log.

Figure 2) shows this example.

Figure 2: E-R diagram with a weak entity set

Nonbinary Relationships

Non-binary relationships can easily be represented. Figure 3) shows an example.

Figure 3: E-R diagram with a ternary relationship

This E-R diagram says that a customer may have several accounts, each located in a specific bank branch, and that an account may belong to several different customers.

The Entity Relationship Diagram
Representation of Strong Entity Sets

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Posted by Akash Kurup

Founder and C.E.O, World4Engineers Educationist and Entrepreneur by passion. Orator and blogger by hobby

Website: http://world4engineers.com