How does blending work in Looker Studio?

A blend joins up to five data sources on shared join keys. You choose the join type — left outer, right outer, inner, full outer or cross — and select which dimensions and metrics come through from each source.

Each source is aggregated to the level of the join keys before being combined, so a one-to-many relationship can duplicate a metric across matched rows and inflate totals.

Always validate a blend by comparing a simple total against the unblended source. Where an upstream join is possible, do that instead — blending is for sources that genuinely cannot be joined, such as two managed APIs.