# 20.3.5 Association Types

### Intended learning outcomes: Differentiate between the 1-to-1 association, the 1-to-n association, the n-to-1 association, and the n-to-n association. Explain various examples of different binary associations.

The association type is an abbreviated characterization of the maximum cardinality of the two roles or access functions.

For a binary association, four association types were defined:

• “One-to-one,” or 1-to-1 association: Both roles are single-valued.
• “One-to-many,” or 1-to-n association: The access function f, or the one role, is multi-valued, and the reverse function, or reverse role, is single-valued.
• “Many-to-one,” or n-to-1 association: The access function f, or the one role, is single-valued, and the reverse function, or reverse role, is multi-valued.
• “Many-to-many,” or n-to-n association: Both roles, or access functions, are multi-valued.

Figure 20.3.5.1 shows examples of various binary associations and also introduces the notation.

Fig. 20.3.5.1       Examples of different binary associations.

The line connecting the two classes stands for the two roles, or access functions. But each role name, together with its minimum and maximum cardinality, is next to the target class of the role. From top to bottom, the first two examples shown are 1-to-1 associations, the third and fourth are 1-to-n associations, the fifth and sixth are n-to-1 associations, and the last four examples are n-to-n associations.

## Course section 20.3: Subsections and their intended learning outcomes

• ##### 20.3 Modeling Information Systems in the Data View and Object View

Intended learning outcomes: Present terms such as object, attribute, object class, view, primary and secondary keys. Explain basic concepts such as association, association role and type. Describe the breaking up of an n-to-n association – in particular a reflexive one – and the association class. Disclose the use of the hierarchical constructs for developing a company-wide generic object model.

• ##### 20.3.1 Object, Attribute, and Object Class

Intended learning outcomes: Differentiate between entity, object, and data record. Differentiate between attribute and data field. Differentiate between object class, file, and table. Explain the class customer as a table in the relational database model.

• ##### 20.3.2 View and Primary and Secondary Keys

Intended learning outcomes: Produce an overview on the view on a class. Differentiate between primary key and secondary key. Describe characteristics for a “good” primary key. Explain the representation of a class in an object-oriented approach (with example) and the representation of an object.

• ##### 20.3.3 Association and (Association) Role

Intended learning outcomes: Differentiate between binary association and reflexive association. Explain Abrial’s access function and the example of the association of the classes “book” and “customer” in library systems. Identify the (association) role.

• ##### 20.3.4 Single-Valued and Multivalued Roles, and Total and Partial Roles

Intended learning outcomes: Differentiate between a single-valued role and a multivalued role. Differentiate between a total and a partial role, or access function.

• ##### 20.3.5 Association Types

Intended learning outcomes: Differentiate between the 1-to-1 association, the 1-to-n association, the n-to-1 association, and the n-to-n association. Explain various examples of different binary associations.

• ##### 20.3.6 Breaking up an n-to-n Association; Association Class

Intended learning outcomes: Explain the association class formed by breaking up an n-to-n association into a 1-to-n association and an n-to-1 association.

• ##### 20.3.7 The Crow’s-Foot Notation and the Common Entity-Oriented Notation

Intended learning outcomes: Describe the commonly used crow’s-foot notation for depicting associations in the entity-oriented approach. Identify the common notation for associations in the entity-oriented approach.

• ##### 20.3.7b The Reidentification Key and the Object-Oriented Notation

Intended learning outcomes: Explain the Representation of associations in an object-oriented form, as well as the concept of the reidentification key.

• ##### 20.3.8 Breaking Up a Reflexive n-to-n Association, Step 1: Duplicate, and Step 2: Breakdown

Intended learning outcomes: Explain Breaking up a reflexive n-to-n association, step 1: Duplicate the class to create its two subclasses, and step 2: Classical breakdown of the n-to-n association.

• ##### 20.3.8b Breaking Up a Reflexive n-to-n Association, Step 3: Merge

Intended learning outcomes: Explain Breaking up a reflexive n-to-n association, step 3: Merge the generating classes.

• ##### 20.3.9 Using the Hierarchical Constructs Based on Basic Ob­ject Classes: The Company-Wide Generic Object Model

Intended learning outcomes: Explain the basic object classes and possible subclasses (specializations) of an industrial company. Describe the concept of a company-wide data model, or generic model.