After some research, I believe that Jasper subreports only work in conjunction with SQL Connections. In my example, I currently only use two different CSV files as a source, but they cannot be merged into one report. My actual goal is a report on MongoDB, but I'm afraid that the driver is not an SQL driver and subreports are also not possible in this context. My example uses the classic example of invoices and invoice items, both from one CSV file each and related rows share a common ID. A report which shows invoices and items together as a list seems to be impossible without SQL. This is a pity. Working examples can only be found in connection with SQL databases. Here is a hint from another source: "... Since a sub-report requires it's own dataset, if you're not using SQL, then your sub-report has no dataset and can't display anything. ..." https://support.illuminateed.com/hc/en-us/community/posts/360013857234-Subreport-The-detail-of-subreport-doesn-t-display-
