Dimension table
A dimension table is a data warehousing concept. It is one of the set of companion tables to a fact table.
The fact table contains business facts or measures and foreign keys which refer to candidate keys (normally primary keys) in the dimension tables.
The dimension tables contain attributes or (fields) used to constrain and group data when performing data warehousing queries.
Over time, the attributes of a given row in a dimension table may change. For example, the shipping address for a company may change. Kimball refers to this phenomena as Slowly Changing Dimensions. Strategies for dealing with this kind of change are divided into three categories:
- Type One - Simply overwrite the old value(s).
- Type Two - Add a new row containing the new value(s), and distinguish between the rows using Tuple-versioning techniques.
- Type Three - Add a new attribute to the existing row.
References
- Kimball, Ralph et al (1998); The Data Warehouse Lifecycle Toolkit, p17. Pub. Wiley. ISBN 0-471-25547-5.
- Kimball, Ralph (1996); The Data Warehouse Toolkit, p100. Pub. Wiley. ISBN 0-471-15337-0.