Joined reports versus a single report type
Key Takeaway
Use a joined report when one report needs data from more than one report type, including a ratio across blocks built with a cross-block custom summary formula. Joined reports can't use bucket fields, cross filters, conditional highlighting or reporting snapshots, so those requirements need a regular report.
A joined report is a report format that combines data from up to 5 standard or custom report types in one report. Each report type appears in its own block, so you can put different kinds of records side by side, such as Accounts, Opportunities and Cases, grouped by the fields they share. In exchange, joined reports can't use some features that regular reports support. To choose between a joined report and a regular report, ask two questions. Does the requirement need more than one report type? Does it need a feature that joined reports give up?
What a joined report is
Because each block has its own report type, one joined report can include up to 5 different report types. You can turn any existing report into a joined report in the report builder.
Each block works much like its own report. For each block, you add regular and summary fields, custom summary formulas and filters, and you sort columns.
Joined reports are based on SQL outer join, so every row from a source block appears in the joined report even when the other blocks have no matching data.
You build a joined report with an Accounts block and a Cases block, grouped by Account Name. The report shows one row per account. Account data sits in the left block, and that account's cases sit in the right block. An account such as Blue Pine Bakery that has no cases still gets its row, with nothing in the Cases block for it.