Relationships Between Tables

Reading Time: 8 Mins | Level: Beginner to Advanced

1. What is Relational Modeling in Power BI?

In real business systems, data is separated into multiple tables (e.g. Customers, Products, Orders). Rather than merging everything into one massive, slow table, Power BI connects tables using Relationships based on matching key columns.

2. Cardinality Types

• *1-to-Many ($1:$):** The gold standard of data modeling. One record in the primary dimension table (e.g., Customer Master) relates to multiple transactions in the fact table (e.g., Sales Orders).
• *Many-to-1 ($:1$):** Same as 1-to-many from the perspective of the fact table.
• 1-to-1 ($1:1$): Rare. Usually indicates two tables that should be merged.
• *Many-to-Many ($:$):* Complex. Can cause ambiguous filter paths and performance degradation. Requires a bridge table.

3. Cross-Filter Direction

• Single (Default & Best Practice): The dimension table filters the fact table. Filters flow in one direction along the arrow.
• Both (Bidirectional): Filters flow in both directions. Use with extreme caution as it can produce unintended calculation results.