Star Schema

In a star schema implementation, OWB stores the dimension data in a single table. Because the same table or view stores data for more than one dimension level, you must specify a dimension key column in the table. The dimension key column is the primary key for the dimension. This column also forms the foreign key reference to the cube.

Each level implements a subset of dimension attributes. By default, the level attribute name is the same as the dimension attribute name. To avoid name conflicts caused by all level data being stored in the same table, OWB uses the following guidelines for naming in a star table:

• If the level attribute name is not unique, OWB prefixes it with the name of the level.

• If the level attribute name is unique, OWB does not use any prefix.

For example, if you implement the Products dimension using a star schema, OWB uses a single table to implement all the levels in the dimension.

Figure: Star Schema Implementation of Products Dimension displays the star schema implementation of the Products dimension. The attributes in all the levels are mapped to different columns in a single table called PRODUCTS. The column called DIMENSION_KEY stores the surrogate ID for the dimension and is the primary key of the table.

For relational or ROLAP dimensions that use a star implementation, you can bind attributes from more than one levels to the same database column. A database column that is bound to attributes from more than one dimension levels is referred to as a shared column. For a Type 2 SCD, you cannot set the level attributes that are bound to a shared column as triggering attributes.