Data dictionary
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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. 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 when discrete logic has been added to data element definitions.
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