Data Dictionaries

After successive levels of data flow diagrams are complete the systems analyst will use them to help catalog data flows, data stores, data structures, and data elements in a data dictionary.

The data dictionary is a reference work of data about data (metadata) compiled by systems analysts to guide them through analysis and design. It collects, coordinates, and confirms what a specific data term means to different people within the organization. The contents of a data dictionary include the descriptions and specifications of system data, but not the actual data itself.

The data dictionary provides documentation, helps eliminate redundancy, provides a starting point for developing screens and reports, can be used to validate the data flow diagram for completeness and accuracy, and is an aid in developing the logic for data flow diagram processes. By compiling a data dictionary, the systems analysts can avoid duplication of efforts, expect better communication between organizational departments sharing a database, make maintenance easier, and utilize it as a consistent standard for data elements.

A data dictionary entry may contain the name of the data item (i.e., what the data item is commonly called in most programs), any aliases for the data item, a brief description in English of the data element, a list of any other data elements related to the specific data item, ranges of acceptable values for the data item.

Naming is of particular importance. When given an opportunity to name components of a system, the systems analyst needs to work at making the names meaningful yet exclusive of other existing names.

For maximum benefit, the data dictionary should be tied into other programs in the system so that when an item is updated or deleted from the data dictionary it is automatically updated or deleted from the database. Otherwise, the data dictionary is reduced to being a type of historic document of the system at some previous point in time. To keep a manual data dictionary up to date and current, it would be necessary for the systems analysts to check the status of the dictionary continuously. It is better to allow the computer to maintain the dictionary through cross-referencing whenever changes are made to the system.

Data dictionaries may be used to create reports, screens, and forms; generate computer program source code; and analyze the system design for completion and to detect design flaws.

A data structure is composed of a group of smaller structures and elements. Algebraic notation is used to represent the data structure. An equal sign means "consists of." A plus sign means "and." Braces are used to identify repetitive elements or a group of elements. Brackets indicate an either-or situation. And, parenthesis are used for optional elements.

Data elements contain information about a specific type of data. Data structures are groups of data elements that are processed together. Data stores are holding places for actual data that has been organized in the form of data structures.