Data dictionary

Data Dictionary

A data dictionary is a collection of descriptions of the data objects or items in a data model for the benefit of programmers and others who need to refer to them. A first step in analyzing a system of objects with which users interact is to identify each object and its relationship to other objects. This process is called data modeling and results in a picture of object relationships. After each data object or item is given a descriptive name, its relationship is described (or it becomes part of some structure that implicitly describes relationship), the type of data (such as text or image or binary value) is described, possible predefined values are listed, and a brief textual description is provided. This collection can be organized for reference into a book called a data dictionary. When developing programs that use the data model, a data dictionary can be consulted to understand where a data item fits in the structure, what values it may contain, and basically what the data item means in real-world terms. For example, a bank or group of banks could model the data objects involved in consumer banking. They could then provide a data dictionary for a bank's programmers. The data dictionary would describe each of the data items in its data model for consumer banking (for example, "Account holder" and ""Available credit").

A data dictionary is a set of metadata that contains definitions and representations of data elements. Within the context of a DBMS, a data dictionary is a read-only set of tables and views. The data dictionary is database in its own dictionary. Amongst other things, a data dictionary holds the following information:

• Precise definition of data elements
• Usernames, roles and privileges
• Schema objects
• Integrity constraints
• Stored procedures and triggers
• General database structure
• Space allocations

One benefit of a well-prepared data dictionary is a consistency between data items across different tables. For example, several tables may hold telephone numbers; using a data dictionary the format of this telephone number field will be consistent.

When an organization builds an enterprise-wide data dictionary, it may include both semantics and representational definitions for data elements. The semantic components focus on creating precise meaning of data elements. Representation definitions include how data elements are stored in a computer structure such as an integer, string or date format (see data type). Data dictionaries are one step along a pathway of creating precise semantic definitions for an organization.

Initially, data dictionaries are sometimes simply a collection of database columns and the definitions of what the meaning and types the columns contain. Data dictionaries are more precise than glossaries (terms and definitions) because they frequently have one or more representations of how data is structured. Data dictionaries are usually separate from data models since data models usually include complex relationships between data elements.

Data dictionaries can evolve into full ontology (computer science) when discrete logic has been added to data element definitions.

See also

• Data element
• Data model
• ISO/IEC 11179
• Metadata registry
• Semantic spectrum
• Web Ontology Language

External links

• Term definition from Webopedia