One-to-one relationship guidance

This article targets you as a data modeler who works with Power BI Desktop. It provides you with guidance on working with one-to-one model relationships. A one-to-one relationship can be created when both tables each contain a column of common and unique values.

Note

An introduction to model relationships is not covered in this article. If you're not completely familiar with relationships, their properties or how to configure them, we recommend that you first read the Model relationships in Power BI Desktop article.

It's also important that you have an understanding of star schema design. For more information, see Understand star schema and the importance for Power BI.

There are two scenarios that involve one-to-one relationships:

• Degenerate dimensions: You can derive a degenerate dimension from a fact table.

• Row data spans across tables: A single business entity or subject is loaded as two (or more) model tables, possibly because their data is sourced from different data stores. This scenario can be common for dimension tables. For example, master product details are stored in an operational sales system, and supplementary product details are stored in a different source.

  It's unusual, however, that you'd relate two fact tables with a one-to-one relationship. That's because both fact tables would need to have the same dimensionality and granularity. Also, each fact table would need unique columns to allow the model relationship to be created.

Degenerate dimensions

When columns from a fact table are used for filtering or grouping, you can consider making them available in a separate table. This way, you separate columns used for filtering or grouping from those columns used to summarize fact rows. This separation can:

• Reduce storage space.
• Simplify model calculations.
• Contribute to improved query performance.
• Deliver a more intuitive Data pane experience to your report authors.

Consider a source table named Sales that stores sales order line reference details in two columns.

The OrderNumber column stores the order number, and the OrderLineNumber column stores a sequence of lines within the order.

In the following image, notice that the order number and order line number columns haven't been loaded to the Sales table. Instead, their values were used to create a surrogate key column named OrderLineNumberID. (The key value is calculated by multiplying the order number by 1000, and then adding the order line number.)

The Sales Order dimension table provides a rich experience for report authors with two columns: Sales Order and Sales Order Line. These particular columns support report designs that need to filter, group, or drill down through orders and order lines.

Because the Sales Order table is derived from the sales data, there should be exactly the same number of rows in each table. Further, there should be matching values between each OrderLineNumberID column.

Row data spans across tables

Consider an example that involves two one-to-one related dimension tables: Product and Product Category. Each table represents imported data and has a SKU (stock-keeping unit) column that contains unique values.

Here's a partial model diagram of the two tables.

The first table is named Product, and it contains three columns: Color, Product, and SKU. The second table is named Product Category, and it contains two columns: Category and SKU. A one-to-one relationship relates the two SKU columns. The relationship filters in both directions, which is always the case for one-to-one relationships.

To help describe how the relationship filter propagation works, the following image reveals some table rows. All examples in this article are based on this data.

The row details for the two tables are described in the following bulleted list:

• The Product table has three rows:
  • SKU CL-01, Product T-shirt, Color Green
  • SKU CL-02, Product Jeans, Color Blue
  • SKU AC-01, Product Hat, Color Blue
• The Product Category table has two rows:
  • SKU CL-01, Category Clothing
  • SKU AC-01, Category Accessories

Notice that the Product Category table doesn't include a row for the product SKU CL-02. We'll discuss the consequences of this missing row later in this article.

In the Data pane, report authors find product-related fields in two tables: Product and Product Category. Let's see what happens when fields from both tables are added to a table visual. In this example, the SKU column is sourced from the Product table.

Notice that the Category value for product SKU CL-02 is BLANK. That's because there's no corresponding row in the Product Category table for this product.

Recommendations

When possible, we recommend you avoid creating one-to-one model relationships when row data spans across model tables. That's because this design can:

• Contribute to Data pane clutter, listing more tables than necessary.
• Make it difficult for report authors to find related fields, because they're distributed across multiple tables.
• Limit the ability to create hierarchies, as their levels must be based on columns from the same table.
• Produce unexpected results when there isn't a complete match of rows between the tables.

Specific recommendations differ depending on whether the one-to-one relationship is intra source group or cross source group. For more information about relationship evaluation, see Model relationships in Power BI Desktop.

Intra source group one-to-one relationship

When a one-to-one intra source group relationship exists between tables, we recommend consolidating the data into a single model table. You can do that by merging the Power Query queries.

The following steps present a methodology to consolidate and model the one-to-one related data.

1. Merge queries: When combining the two queries, give consideration to the completeness of data in each query. If one query contains a complete set of rows (like a master list), merge the other query with it. Set the merge transformation to use a left outer join, which is the default join type. This join type ensures you'll keep all rows of the first query, and supplement them with any matching rows of the second query. Expand all required columns of the second query into the first query.

   

2. Disable query load: Be sure to disable the load of the second query. This way, it won't load its result as a model table. This configuration reduces the data model storage size, and helps to unclutter the Data pane.

   In our example, report authors now find a single table named Product in the Data pane. It contains all product-related fields.

3. Replace missing values: If the second query has unmatched rows, null values appear in the columns introduced from it. When appropriate, consider replacing null values with a token value. Replacing missing values is especially important when report authors filter or group by the column values, as BLANKs could appear in report visuals.

   In the following image, notice that the category for product SKU CL-02 now reads [Undefined]. In the query, null categories were replaced with this token text value.

   

4. Create hierarchies: If relationships exist between the columns of the now-consolidated table, consider creating hierarchies. This way, report authors will quickly identify opportunities for report visual drilling.

   In our example, report authors now can use a hierarchy that has two levels: Category and Product.

   

If you like how separate tables help organize your fields, we still recommend consolidating into a single table. You can still organize your fields, but by using display folders instead.

In our example, report authors can find the Category field within the Marketing display folder.

Should you still decide to define one-to-one intra source group relationships in your model, when possible, ensure there are matching rows in the related tables. As a one-to-one intra source group relationship is evaluated as a regular relationship, data integrity issues could surface in your report visuals as BLANKs. (You can see an example of a BLANK grouping in the first table visual presented in this article.)

Cross source group one-to-one relationship

When a one-to-one cross source group relationship exists between tables, there's no alternative model design—unless you pre-consolidate the data in your data source. Power BI will evaluate the one-to-one model relationship as a limited relationship. Therefore, take care to ensure there are matching rows in the related tables, as unmatched rows are eliminated from query results.

Let's see what happens when fields from both tables are added to a table visual, and a limited relationship exists between the tables.

The first table visual, which uses a cross-source group relationship, displays two rows only. Product SKU CL-02 is missing because there's no matching row in the Product Category table. The second table visual, based on a single consolidated table in the model, displays three rows.

Related content

For more information related to this article, check out the following resources:

• Model relationships in Power BI Desktop
• Understand star schema and the importance for Power BI
• Relationship troubleshooting guidance
• Questions? Try asking the Fabric Community
• Suggestions? Contribute ideas to improve Fabric