There is a problem since firebird version 1.5.3. If you have
used :params in GROUP BY clausule, firebird generates error :

Dynamic SQL Error.
SQL error code = -104.
Invalid expression in the select list (not contained in either an
aggregate function or the GROUP BY clause).

For example, this select works up to 1.5.2 version, but not since
1.5.3 :

select
COUNT(*),
substr(code,1,:Par)
from
STREDISKA
group by
substr(code,1,:Par)

if you change :Par by constant number, it works:

select
COUNT(*),
substr(code,1,3)
from
STREDISKA
group by
substr(code,1,3)

You can try it on this table:
/***********************************/
/* Table: STREDISKA */
CREATE TABLE STREDISKA (
ID INTEGER NOT NULL,
CODE VARCHAR (50) CHARACTER SET WIN1250 COLLATE WIN1250);
/* Primary keys definition */
ALTER TABLE STREDISKA ADD CONSTRAINT PK_STREDISKA PRIMARY KEY (ID);
/* Indices definition */
CREATE UNIQUE INDEX PK_STREDISKA ON STREDISKA (ID);

INSERT INTO STREDISKA
(ID,CODE)
VALUES
(1,'001234');
INSERT INTO STREDISKA
(ID,CODE)
VALUES
(2,'001564');
INSERT INTO STREDISKA
(ID,CODE)
VALUES
(3,'002666');
INSERT INTO STREDISKA
(ID,CODE)
VALUES
(4,'003555');
/**********************************/




Best regards

Bc. ¼uboš Okruhlica
Programmer, analyst