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How Joined Reports Work
A joined report can contain data from multiple standard or custom report types. You can add report types to a joined report if they have relationships with the same object or objects. For example, if you have a joined report that contains the Opportunities report type, you can add the Cases report type as well because both have a relationship with the Accounts object.
A joined report consists of up to five report blocks, which you add to the report to create multiple views of your data. For each block, you can add regular and summary fields, create standard and cross-block custom summary formulas, apply filters, and sort columns. You apply groupings across all blocks in the report, and can add up to three groupings to the blocks, the same as for the summary format. You can also add a chart to a joined report.
Each joined report has a principal report type. By default, the principal type is the first one added to the report. For example, if you create the joined report by selecting the Opportunities report type, and then add the Cases type, the Opportunities type is the principal report type.
The principal report type controls how common fields are named. Some common fields have different names or appear in different sections in different report types.
When a joined report contains multiple report types,
some fields are identified as common fields. A field is a common field if
it’s shared by all report types or if all report types share a lookup relationship to
the field. Common fields can be used to group report blocks. In Lightning Experience, common
fields are identified by the 
icon. In Salesforce Classic, common fields appear
in the Common Fields area in the Fields pane.
Joined reports are based on SQL outer join. Each row in the source blocks has a corresponding row in the joined report, regardless of whether all the blocks that make up the joined report have matching data.
