Relationships
Relationships play a central role in the analytical behavior and performance of the dataset. Based on the filters applied at the report layer and the DAX expressions contained in the measures, relationships determine the set of active rows for each table of the model to be evaluated. It's critical that the dataset designer understands how relationships drive report behavior via cross-filtering and the rules that relationships in Power BI must adhere to, such as uniqueness and non-ambiguity.
Uniqueness
Relationships in Power BI Data Models are always defined between a single column from each of the two tables. One of these two columns must uniquely identify the rows of its table, such as the Currency Key column from the Currency table in the Fact-to-dimension relationships section earlier in this chapter. Power BI will throw an error message if a row with a duplicate value for the relationship column is attempted to be loaded to the one side of the relationship, as shown in...
