Inverted-L charts

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Inverted-L charts

27.1 Purpose

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

27.2 Strengths, weaknesses, and limitations

An inverted-L chart is a clear, easy-to-visualize, graphical model of a data structure. The inverted-L model lacks many of the necessary details that must 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 inverted-L chart is prepared in conjunction with an entity-relationship model, the entity-relationship model (Chapter 26) is generally prepared first.

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

27.4 Concepts

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

Figure 27.1 shows two examples of inverted-L diagrams. The entity name (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 that the key field (or fields) is clearly marked.

The entity’s links (or relationships) are listed below the attributes. For example, note the linkSupplied by supplier under Inventory. Move down to the second inverted-L chart and find the link that reads Supplies inventory. The same link appearing in inverse form under two entities defines the relationship between them. Some analysts add cardinality limits to the links; the greater than (>) symbol implies a “many to” relationship.

27.5 Key terms
Attribute
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 system’s 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 Software

The inverted-L diagrams in this chapter were prepared using Visio. Other graphing tools, such as Micrografx’s Flowcharter provide comparable support. Additionally, many tools for creating entity-relationship models also support inverted-L charts.

27-01
Figure 27.  Two inverted-L diagrams.

27.7 References
1.  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, Blackwell Scientific, Oxford, U.K., 1990.

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