Thanks for responding.
I knew of TOP. But I didn't want one row. Which is why I wrote
return a single value from a row
.
Anyways the SSRS dialogs for setting that parameter from a query expected a single value and gave me the option of just selecting that value/column from the resulting data (one row). Which TOP(1) provided.
So I didn't need to make the query return a single value as I'd thought. A single row sufficed.