Database

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. (See Elmasri & Navathe, chapter 21.)
  • 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 2.11).

  
Figure 2.11: 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 2.12) shows this example.

  
Figure 2.12: E-R diagram with a weak entity set
 

Nonbinary Relationships

Non-binary relationships can easily be represented. Figure 2.13) shows an example.

  
Figure 2.13: 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.