What is data blending in Tableau?

Data blending combines data from two separate data sources into one view, at the level of the visualization itself, rather than at the query level like a join does.

• One data source is designated the primary, the other the secondary; the primary's row-level data effectively drives the view.
• Tableau connects them using one or more common fields (a linking field), aggregating the secondary source's data up to match the primary's level of detail.
• Because blending aggregates the secondary source first, secondary-source measures show up already summarized — you can't drill into their row-level detail the way you can with the primary source.

Primary:   Sales (by Region, Month)
Secondary: Marketing Spend (by Region, Month)
-> linking field: Region + Month
-> Marketing Spend appears aggregated to that grain in the view

Blending is the go-to option when two data sources can't be joined directly — for example, they live in entirely different databases, or one is a live connection and the other an extract — but it comes with the aggregation trade-off that a proper join or unioned extract avoids.

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