Blending with Multiple Secondary Data Sources Returns Incorrect Values

Issue

When one secondary data source is blended on a higher level of granularity than the view, and another secondary data source is not blended on this field, the other secondary data source might return duplicate values.

Environment

Tableau Desktop

Resolution

Try one of the following options, as appropriate for your workbook:

It may be possible to cross database join instead of blending. See Join Your Data for more information.
Filter each of the data sources independently on the more granular dimensions.
Make sure that you are blending on dimensions present in the view or on all of the same dimensions across secondary data sources.

Cause

A primary data source is partitioned behind the scenes by the fields that it has been linked on. So if a dimension not present in a view is blended on one secondary data source but not a second secondary data source, the view is partitioned and the blend duplicates values for the second secondary data source.