A data dictionary is a description of data in business terms, also including information about the data such as data types, details of structure, and security restrictions. Unlike business glossaries, which focus on data across the organization, data dictionaries support data warehouses by defining how to use them. The content of the data dictionary often originates from the logical data model. Data dictionaries typically result in higher-quality metadata due to a disciplined and systematic approach to managing definitions.
While data dictionaries come in many different shapes and sizes, they all describe the ingredients and steps to create relevant reports from a relational database. The International Organization for Standardization (ISO) classifies data dictionaries in three categories:
- Business Concepts: Entries with business semantic meaning, including Business Associations, Components, Constraints, Elements, and Roles. For example, a Display Name would contain business concepts to be relevant.
- Data Types: Specifications about the valid values of a Business Element. In the example below, average length of RNA sequences is an integer, making decimal entries (e.g., 3.4) invalid.
- Message Concepts: Message Components, Constraints, and Elements that uniquely define messages. They can be characterized by fixed selections. For example, standard deviation, in the table below, can be 0.0-9999. Values outside of this range are not accepted in this database.
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Other Definitions of a Data Dictionary Include:
- “A file that defines the format of data in an ASCII flat file. It defines the field names, their order, their data type and the byte positions they occupy in the file.” (IBM Knowledge Center)
- A resource providing “the ingredients and steps needed to create relevant business reports.” (Michelle Knight)
- A “collection of names, definitions, and attributes about data elements that are being used or captured in a database.” (UC Merced Library)
- “An in-house resource with terms in one place. Most importantly this reference guide facilitates communication with business resulting in better understood definitions.” (Amber Lee Dennis).
- “Defines the structure of the database itself and is used in database control and maintenance.” (Business Dictionary)
Businesses Use Data Dictionaries to:
- Ensure agreement between the business-facing content and technical-facing physical data.
- Reduce the risk of downstream errors and rework.
- Provide useful reports.
- Assure smoother database upgrades.
- Guarantee more meaningful metadata.
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