Using bind variables (in & out) with dynamic sql

Question

I got a table that holds pl/sql code snippets to do validations on a set of data. what the code basically does is receiving a ID and returning a number of errors found.
To execute the code I use dynamic sql with two bind variables.
When the codes consists of a simpel query, it works like a charm, for example with this code:
BEGIN
   SELECT COUNT (1)
   INTO :1
   FROM articles atl
   WHERE ATL.CSE_ID = :2 AND cgp_id IS NULL;
END;however when I get to some more complex validations that need to do calculations or execute multiple queries, I'm running into trouble.
I've boiled the problem down into this:
DECLARE
   counter   NUMBER;
   my_id     NUMBER := 61;
BEGIN
   EXECUTE IMMEDIATE ('
      declare
         some_var number;
      begin
      select 1 into some_var from dual
      where :2 = 61;
      :1 := :2;
      end;
      USING OUT counter, IN my_id;
   DBMS_OUTPUT.put_line (counter || '-' || my_id);
END;this code doesn't really make any sense, but it's just to show you what the problem is. When I execute this code, I get the error
ORA-6537 OUT bind variable bound to an IN position
The error doesn't seem to make sense, :2 is the only IN bind variable, and it's only used in a where clause.
As soon as I remove that where clause , the code will work again (giving me 61-61, in case you liked to know).
Any idea whats going wrong? Am I just using the bind variables in a way you're not supposed to use them?
I'm using Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit

Answer 1

Correction. With execute immediate binding is by position, but binds do not need to be repeated. So my statement above is incorrect..
You need to bind it once only - but bind by position. And the bind must match how the bind variable is used.
If the bind variable never assigns a value in the code, bind as IN.
If the bind variable assigns a value in the code, bind as OUT.
If the bind variable assigns a value and is used a variable in any other statement in the code, bind as IN OUT.
E.g.
SQL> create or replace procedure FooProc is
  2          cnt     number;
  3          id      number := 61;
  4  begin
  5          execute immediate
  6  'declare
  7          n       number;
  8  begin
  9          select
10                  1 into n
11          from dual
12          where :var1 = 61;       --// var1 is used as IN
13 
14          :var2 := n * :var1;     --// var2 is used as OUT and var1 as IN
15          :var2 := -1 * :var2;    --// var2 is used as OUT and IN
16  end;
17  '
18          using
19                  in out id, in out cnt;  --// must reflect usage above
20 
21          DBMS_OUTPUT.put_line ( 'cnt='||cnt || ' id=' || id);
22  end;
23  /
Procedure created.
SQL>
SQL> exec FooProc
cnt=-61 id=61
PL/SQL procedure successfully completed.
SQL>