Preparation:

with dm.PRPatientenQuery do
 begin
   sql.add('select * from Patienten where PatientenID in');
   sql.add('(select PatientenID from Anforderungen where Ambulant = :Ambulant)');
   prepare;
 end;

Execution:

 with dm.PRPatientenQuery do
 begin
   close;
   ParamByName('Ambulant').asBoolean := true;
   open;
   ShowMessage(inttostr(dm.PRPatientenQuery.recordcount));
   close;
   ParamByName('Ambulant').asBoolean := false;
   open;
   ShowMessage(inttostr(dm.PRPatientenQuery.recordcount));
 end;