In data warehousing and database design, dimension role play refers to a situation where a single dimension table is related to a fact table multiple times, with each relationship representing a different context or "role" for the dimension.
Understanding Role-Playing Dimensions
Based on the provided reference:
A table with multiple relationships between itself and another table is known as a role-playing dimension.
Essentially, one dimension table plays different roles in relation to a fact table. Instead of creating separate, identical dimension tables for each role, the same dimension table is used, but joined to the fact table using different foreign keys.
This is a common technique to avoid redundancy and maintain consistency across the different contexts or roles the dimension might represent in the data.
Example: Time Dimension in Sales
A classic example of a role-playing dimension is the Time dimension in a Sales fact table, as illustrated in the reference:
For example, the Sales fact in the following example has multiple relationships to Time on the keys Order Day, Ship Day, and Close Day.
In this scenario, the same Time dimension table is used to represent:
- The Order Date (when the order was placed).
- The Ship Date (when the order was shipped).
- The Close Date (when the order was closed or completed).
Each of these corresponds to a different column (foreign key) in the Sales fact table that links back to the primary key of the Time dimension table.
To effectively query data using a role-playing dimension, you typically create views or aliases for the dimension table for each role you want to use. This makes queries clearer and allows you to filter or group by the same time attributes (like year, month, day of the week) for different dates associated with a single sales record.
The reference also suggests a design approach:
Create a table for each role that you want the Time table to play.
While the core concept of role-playing involves using the same dimension table multiple times, in some implementation scenarios, particularly for simpler reporting tools or performance reasons, you might materialize separate physical tables or views based on the original dimension for each role, although the logical model still reflects a single underlying dimension playing multiple roles. However, the primary definition centers on the logical concept of multiple relationships from one dimension table.
