Executing dynamic queries without functions / procedures

Is it possible to execute a dynamic query similarly to MS SQL, like so:
IF EXISTS(SELECT * FROM Table WHERE Condition)
DO SOMETHING

I would like to use the PL language in conjunction with conditional statements, outside of a procedure or function.  This doesn't seem to be possible from what I've read so far and I wanted to confirm.

Also, if it's not natively possible - is there some kind of hack?  For example, creating temporary procedures to establish the same result, then removing them periodically?

Thanks much.