In Power BI, Reference and Duplicate queries are two essential techniques in Power Query Editor that help you manage and transform your data efficiently.
**Duplicate Query:**
When you duplicate a query, Power BI creates an entirely independent copy of the original query. This new query starts fres…In Power BI, Reference and Duplicate queries are two essential techniques in Power Query Editor that help you manage and transform your data efficiently.
**Duplicate Query:**
When you duplicate a query, Power BI creates an entirely independent copy of the original query. This new query starts fresh from the data source and includes all the transformation steps from the original. The duplicated query has no connection to the original query - changes made to one will not affect the other. This approach is useful when you need to create a completely separate data pipeline that starts from the same source but will undergo different transformations. However, duplicating queries means the data source is accessed multiple times, which can impact performance and refresh times.
**Reference Query:**
A reference query creates a new query that uses the output of an existing query as its starting point. Instead of going back to the original data source, the referenced query builds upon the results of the source query. This creates a dependency relationship between the queries. Any modifications to the original query will automatically flow through to the referencing query. This method is more efficient because the base transformations are performed once, and subsequent queries can add additional steps on top of those results.
**Key Differences:**
- Duplicate queries are independent; reference queries are dependent
- Duplicate queries reload from the source; reference queries use cached results
- Reference queries promote better performance and reduce redundant processing
- Reference queries are ideal for creating multiple outputs from a common base transformation
**Best Practices:**
Use reference queries when you need multiple variations of the same dataset with shared initial transformations. Use duplicate queries when you require completely independent data pipelines. Understanding these concepts helps optimize your data model, reduce refresh times, and maintain cleaner, more organized query structures in your Power BI solutions.
Reference and Duplicate Queries in Power BI
Why Reference and Duplicate Queries Are Important
Understanding the difference between reference and duplicate queries is crucial for the PL-300 exam because it directly impacts data model efficiency, query performance, and resource management. Making the wrong choice can lead to unnecessary data refreshes, increased memory consumption, and slower report performance.
What Are Reference and Duplicate Queries?
Duplicate Query: A duplicate query creates an independent copy of the original query. The new query has no connection to the source query, meaning changes to the original will not affect the duplicate. The duplicate query re-executes all transformation steps from the data source independently.
Reference Query: A reference query creates a new query that depends on the output of an existing query. It uses the result of the source query as its starting point. Changes to the source query will propagate to all queries that reference it. This creates a dependency chain between queries.
How They Work
Creating a Duplicate: 1. Right-click on a query in the Queries pane 2. Select 'Duplicate' 3. A new independent query is created with all the same steps 4. Both queries load data separately from the source
Creating a Reference: 1. Right-click on a query in the Queries pane 2. Select 'Reference' 3. A new query is created that starts from the output of the original 4. The referenced query must complete before dependent queries can run
Key Differences
- Dependencies: Reference queries create dependencies; duplicates do not - Data Source Calls: Duplicates make separate calls to the data source; references build upon existing query results - Maintenance: References are easier to maintain since changes cascade; duplicates require individual updates - Query Folding: Duplicates can fold independently; references may break folding if the source query cannot fold
When to Use Each
Use Reference When: - You need multiple tables derived from the same base transformation - You want to create a staging query pattern - You need to maintain consistency across related queries - Building dimension and fact tables from common source data
Use Duplicate When: - You need completely independent queries - You plan to modify the base transformations differently - You want to avoid dependency chains - Testing different transformation approaches
Exam Tips: Answering Questions on Reference and Duplicate Queries
1. Look for keywords: Questions mentioning 'dependency', 'cascading changes', or 'shared transformations' typically point to reference queries
2. Consider performance scenarios: If a question asks about reducing data source calls or optimizing refresh times, reference queries are usually the better choice
3. Staging query pattern: When questions describe creating a base query that feeds multiple output tables, think reference queries
4. Independence clues: If the scenario requires queries to be modified independently or have different refresh schedules, duplicate is likely correct
5. Watch for traps: Questions may try to confuse you by describing a scenario where duplicate seems correct but performance requirements suggest reference
6. Remember the M code: Reference queries show the source query name in their first step; duplicates show the full source connection
7. Query dependencies view: Know that you can see reference relationships in the Query Dependencies view under the View tab
8. Disabled load scenarios: Questions about staging queries often combine references with disabling load on the base query to optimize the data model