PL-300 Star Schema Practice Question
A data analyst is designing a star schema in Power BI. The model includes a table named 'Orders' with columns: OrderID, CustomerID, OrderDate, ProductID, Quantity, and SalesAmount. Which column should NOT be included in the fact table to maintain a proper star schema?
⚠ Common exam trap
Candidates often assume that all ID columns belong in the fact table. However, natural keys (like OrderID) should reside in dimension tables; the fact table should contain a surrogate key to reference them. CustomerID, on the other hand, is a foreign key that is correctly placed in the fact table.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
OrderID
In a proper star schema, fact tables should contain quantitative measures and foreign keys to dimension tables. Columns like CustomerID and ProductID serve as foreign keys linking to dimension tables, so they should remain in the fact table. OrderID is a natural key that typically belongs in an Order dimension table; the fact table should use a surrogate OrderKey instead. Including OrderID directly would duplicate dimensional data and reduce modeling flexibility.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Quantity
Why it's wrong here
Quantity is a numeric, whole-number measure that represents the count of units sold in a transaction, making it a classic fact that should be stored in the fact table. This measure supports aggregations such as total units sold or SUM of Quantity, and it is also used in calculations like average selling price. Because it is a transactional metric, not an identifier or descriptive attribute, it belongs in the fact table alongside other measures such as SalesAmount. Hence, Quantity is not the correct answer.
- ✗
SalesAmount
Why it's wrong here
SalesAmount is a quantitative, monetary measure that the fact table is designed to store. In star schema design, numeric, additive metrics like revenue belong in fact tables so they can be aggregated across dimension attributes. Since SalesAmount is a measure rather than a key or descriptive attribute, placing it in a dimension table would be nonstandard. Therefore, including it in the fact table is appropriate, making SalesAmount an incorrect answer to this question.
- ✗
CustomerID
Why it's wrong here
CustomerID is a surrogate key or foreign key that links the fact table to the Customer dimension table, enabling star schema joins and filter propagation. Since fact tables must contain foreign keys to reference dimensions, CustomerID is correctly placed in the fact table as part of the relationship structure. It is not a measure and it is not a natural business key; it is a reference key. So CustomerID is an appropriate fact table column, making this a wrong choice for the question.
- ✓
OrderID
Why this is correct
OrderID is a natural business key that identifies an order, not a numeric measure or a surrogate foreign key. In a star schema, natural keys and descriptive attributes should live in the appropriate dimension table, such as an Order dimension, so that the fact table contains only relationship keys and measures. Including OrderID directly in the fact table violates star schema normalization and reduces maintainability. Thus, OrderID is the item that should NOT be placed directly in the fact table.
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