In blending, you only get the aggregate values of measures. That said, if you try to bring in dimensions, it would only display a *. That means there is more than 1 element on secondary DS. Because, a blending is not a join, to bring in all the dimension records. However, if you have one-one relationship, it would bring that first element only.
If you are dealing with one-many between primary and secondary, you cannot use blending. Your only option is to Join.
1) You could either go to the Database and join them and bring it back as a flat table.
2) Or you could try your luck with a tableau join.
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You are trying to use a field from your secondary data source as a dimension. You cannot do that with data blending. Data blending is used to bring in aggregates from a secondary data source, and that implies that the secondary data source cannot be used to further segment the data. All of your dimensions must be in the primary data source, and the secondary data source is only used to bring in aggregate measures that correspond to the level of detail dictated by the primary data source.
For your application, you will need to JOIN your data instead, thereby creating a single data source that incorporates both tables. Joins happen at the row level, rather than the aggregate level, so dimensions from both data sources can be used to segment the data on your view.
If your tables come from the same connector (e.g. the same Excel file or the same RDBMS), you can directly join them in Tableau 9 or 10 (or even earlier versions, probably).
If your tables come from two completely different data sources, you can use cross data source joins, introduced with Tableau 10.0. If you do not have Tableau 10, you are unfortunately stuck with data blending. In that case, you will need to either (1) restructure your data sources so that all of your dimensions appear in your primary data source, or (2) bite the bullet and rework your sheets to swap the primary and secondary data sources, like you showed in your example.
I hope this helps.
Thanks you for the detail explanation.