attached is a sample report with a couple of group level sql expressions where sql expressions with selects can be handy. however, as Dell mentioned, they were neve designed to do this (aka not supported) and don't work in all db's. sql server, access, hana, mysql are db's where you can get this to work.
note the syntax where an alias is used for the table in the select and a where clause is defined with the main report table to the alias. if you get errors at this part, then chances are that getting this to work will be small.
to use the sample, extract the contents and change the .txt extension to .rpt.
-jamie