I know I've seen this in the past but I can't recall the solution.
I have a custom element in TR_QUERY_ELEMENT which I know (in some cases) will return more than one row. (Specifially, I am pulling a concatenation of first and last name from tx_alias_cust, based on the alias type). Some people have multiple aliases of the chosen type. And, as expected, the Query Elelment is marked to indicate that it could return multiple rows.
The problem is, when I try to run "Execute An Output Set" I get a SQL error stating that a subquery has returned more than one row. I'm missing a piece somewhere. Why doesn't it complete the report while simply returning multiple rows when it needs to?
Found the answer to the problem. I had forgotten to use "!." in front of the field names I was pulling.
Its always the little things.