You must configure join conditions for all the data sources in the join view.
When you configure a join condition, select the data source in the table definition list and specify the left and right fields of the join condition. Data Validation Option displays the alias names of the data sources. The left field of the join condition consists of the output fields from the alias name you choose. The right field consists of the output fields of the data source for which you configure the join condition.
You can create a join condition with any of the data sources in the previous rows in a join view. You can create multiple join conditions for the same data source. You cannot save a join view unless you create at least one valid join condition for all the data sources.