When to use joins versus Blending in Tableau?

Join when the tables are reachable from a single connection and the granularity will not fan out. One query, row-level access to every field, and the result can be published as a reusable data source.

Blend when the sources cannot be joined — separate systems, or connectors like Google Analytics that Tableau manages as their own source — or when the two sides sit at different levels of detail.

That second case is the one worth explaining. Joining daily transactions to monthly targets repeats the target across every day of the month and inflates any total. A blend aggregates each source to the view level first, so both figures stay correct.

Also worth knowing that relationships, introduced in 2020.2, cover much of this middle ground. They keep tables at their own granularity and let Tableau decide the join at query time, which removes many of the situations where blending used to be the only answer.