How to Fix Data Blending Error with Logical Tables in Tableau

"Data sources that contain logical tables cannot be used as secondary data sources for data blending."

This error occurs because Tableau's newer data model, Relationships (logical layer), conflicts with the older method of combining data known as Data Blending. In a data blend, the secondary data source must be a single, flat table structure. Therefore, any data source containing logical tables cannot act as a secondary data source.

Here are three practical ways to resolve this issue.

1. Use the Data Source as the 'Primary' Source
The restriction only applies when the data source with logical tables acts as the secondary source. By dragging a field from this data source into your sheet first, you establish it as the Primary Data Source (blue checkmark) and avoid the error.

2. Switch from Relationships to 'Physical Joins'
If your business logic requires this specific data source to be the secondary one, go to the Data Source tab and double-click the logical table. Connect your tables using traditional Physical Joins to merge them into a single, flat table. It will now function perfectly as a secondary data source.

3. (Recommended) Use 'Cross-Database Relationships' Instead of Blending
This is the most performance-friendly solution. Instead of blending at the view level, click the 'Add' connection button in the Data Source tab to bring in your second database and establish a Cross-Database Relationship directly. This approach ensures a much more stable and flexible dashboard.