The Joining term is used when you are combining data from the same source, for example, worksheet in an Excel file or tables in an Oracle database. While blending requires two completely defined data sources in your report.

In Tableau, joining and blending are both methods used to combine data from multiple sources, but they have distinct differences:

Joining:
- Joining in Tableau involves combining data from different tables within the same data source based on a common field or key.
- It creates a single, unified data source by appending rows from one table to another based on matching values in the specified fields.
- Joining is typically used when the data is structured and resides in a relational database or Excel sheets where relationships can be established.
Blending:
- Blending in Tableau involves combining data from multiple data sources, usually from different databases or files, by linking them together in the same worksheet or dashboard.
- Blending is performed at the visualization level rather than at the data source level.
- Unlike joining, blending does not require a predefined relationship between the data sources. Instead, it allows for on-the-fly data integration based on common dimensions or fields used in the visualization.
- Blending is commonly used when joining is not feasible due to different data sources or when the data sources cannot be altered to establish a direct relationship.

In summary, joining is used to combine data within the same data source based on predefined relationships, while blending is used to integrate data from multiple sources in a visualization without requiring a predefined relationship at the data source level. Each method has its use cases depending on the nature of the data and the requirements of the analysis or visualization.