Beta Notes
Editor - Debugger
Execute PL/SQL - Set Parameters
Input argument values can be set to SELECT statements. Set the argument’s type to expression and its value to a SELECT that returns a single column and single row.
Dennis,
I'm on Beta 53 now and I wanted to try this feature.
So I entered
select *
from dual
where dummy = :x;
Then pressed F9 and got the following pop-up:
I can see only the options to specify a literal or an environment variable.
No expression option 
Am I missing something?
