The Key Elements of Database Dictionaries
A database dictionary, also known as a metadata repository or data dictionary, is a store of the structure of a database. It defines how data is stored and accessed. The database dictionary has to be held in the same database as the data, otherwise the Database Management System, or DBMS, would be unable to use it to create the structure that hold the data of the database. The dictionary can also be held in paper form or developed in external applications for access by the technicians who work with the database. There are basic elements of data structure that needs to be held in the database dictionary.
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Structure
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Each piece of data needs to be kept separate so that one can be distinguished from another. Data elements usually are identified by two dimensions -- a record structure and a column structure. A record is a related group of instances of fields, or a table of rows. The columns are the definitions of each field in a record. A data dictionary holds more information about the columns in the database than about the rows, as the rows are formed by data, which is not the direct responsibility of the database dictionary.
Data Types
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Data types are part of the definition of a column in the database. This specifies what type of data should be expected in that field. The basic types of data divide between numeric and alphanumeric. Each of these two categories are usually further divided. Numeric fields should be defined to state whether decimal places are allowed in the field. Alphanumeric fields definitions include whether the field should hold one character, or a string of many character. Field sizes can also be applied to numeric columns. Alphanumeric fields can have formats imposed on them, such as date formats or a restriction in the possible values that can be held in the fields.
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Data Constraints
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The distinction between constraints and data types can often be blurred. For example, the restrictions on the width of a column is a column constraint. Data constraints do not form part of a column definition. Constraints are the rules that enable relationships between data held in the structure of the database. They ensure that the data held in one table will not match with multiple records in another or cause records to be missed out of a link because it lacks essential data. This is called “data integrity” and is enforced by constraints that forbid some columns to contain null values and also enforce uniqueness on the contents of specified columns.
Other Elements
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Databases store other objects that are not purely intended for holding or defining data, but form an integral part of the operations of the database. Examples of these elements are indexes, which speed up access to the data, and views, which are prewritten queries that represent themselves as database structures to the user. A schema is a user account that holds data objects and also enforces security measures by restricting the actions that certain users can perform. These elements are part of a database's operational structure without being key elements of the data dictionary.
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References
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