Is there a way in to just run a query once to select into a variable, considering that the query might return nothing, then in that case the variable should be null.
From all the answers above, Björn's answer seems to be the most elegant and short. I personally used this approach many times. MAX or MIN function will do the job equally well. Complete PL/SQL follows, just the where clause should be specified.
declare v_column my_table.column%TYPE;
begin
select MIN(column) into v_column from my_table where ...;
DBMS_OUTPUT.PUT_LINE('v_column=' || v_column);
end;
COALESCE
will always return the first non-null result. By doing this, you will get the count that you want or 0:
select coalesce(count(column) ,0) into v_counter from my_table where ...;