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chapter twenty-seven

Inverted-L charts

William S. Davis and David C. Yen

Contents

27.1 Purpose27.2 Strengths, weaknesses, and limitations27.3 Inputs and related ideas27.4 Concepts

27.5 Key terms27.6 Software27.7 References

27.1 PurposeAn inverted-L chart is a tool for graphically representing a data structure.Inverted-L charts are often used with entity-relationship diagrams to docu-ment the attributes that make up an entity. The completed inverted-L chartsrepresent a preliminary set of logical data structures.

27.2 Strengths, weaknesses, and limitationsAn inverted-L chart is a clear, easy-to-visualize, graphical model of a datastructure. The inverted-L model lacks many of the necessary details thatmust be recorded in the data dictionary, however.

27.3 Inputs and related ideas

Before an inverted-L chart can be constructed, the data elements (or attributes)that make up the data structure or entity must be known (Part II). If an

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inverted-L chart is prepared in conjunction with an entity-relationshipmodel, the entity-relationship model (Chapter 26) is generally prepared first.

Alternatives for documenting data structures include the data dictio-nary (Chapter 25) and Warnier-Orr diagrams (Chapter 33). Data conceptsare discussed in Chapters 43, 44, and 45.

27.4 ConceptsAn inverted-L chart is a tool for graphically representing a data structure.Inverted-L charts are often used with entity-relationship diagrams to docu-ment the attributes that make up an entity. The completed inverted-L chartsrepresent a preliminary set of logical data structures.

Figure 27.1 shows two examples of inverted-L diagrams. The entityname (or data structure name) appears at the top of the imaginary upside-down letter L. Attributes are listed under the entity name, and some analysts

like to include the data type or a picture clause for each attribute. Note thatthe key field (or fields) is clearly marked.The entitys links (or relationships) are listed below the attributes. For

example, note the link Supplied by supplier under Inventory. Move down tothe second inverted-L chart and find the link that reads Supplies inventory.The same link appearing in inverse form under two entities defines the rela-tionship between them. Some analysts add cardinality limits to the links; thegreater than (>) symbol implies a many torelationship.

27.5 Key termsAttribute A property of an entity.Cardinality A measure of the relative number of occurrences of two

entities.Data element An attribute that cannot be logically decomposed.Data structure A set of related data elements; a composite.Entity An object (a person, group, place, thing, or activity) about

which data are stored.Entity-relationship diagram A diagram that shows how a systems

primary data entities are related.Inverted-L chart A tool for graphically representing a data structure.Key The attribute or group of attributes that uniquely distinguishes

one occurrence of an entity.Relationship A link between two entities or data structures.

27.6 SoftwareThe inverted-L diagrams in this chapter were prepared using Visio. Othergraphing tools, such as Micrografxs Flowcharter provide comparable sup-

port. Additionally, many tools for creating entity-relationship models alsosupport inverted-L charts.

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Figure 27.1 Two inverted-L diagrams.

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27.7 References1. Barker, R., Case Method: Entity Relationship Modelling, Addison-Wesley, Reading,

MA, 1990.2. Davis, W. S., Business Systems Analysis and Design, Wadsworth, Belmont, CA,

1994.

3. McDermid, D. C.,Software Engineering for Information Systems,

BlackwellScientific, Oxford, U.K., 1990.

1999 by CRC Press LLC