SP like this one compiles and works pretty well (it returns the
proper results):

COMMIT WORK;
SET AUTODLL ON;
SET TERM ^;
CREATE PROCEDURE SPS_PROC$GET_RESULTS
(ACREW_ID SMALLINT,
ASALESREP_CODE VARCHAR(5))
RETURNS
(ASUM_PRICE DECIMAL(25, 4))
AS
BEGIN
SELECT SUM(PRICE) FROM MY_DATA
WHERE ((:ACREW_ID IS NULL) OR (CREW_ID = :ACREW_ID)) AND
((:ASALESREP_CODE IS NULL) OR (SALESREP_CODE = :ASALESREP_CODE))
INTO :ASUM_PRICE;
SUSPEND;
END
^
COMMIT WORK ^
SET TERM ; ^

The SP is flexible, in the case I will get the summary result only
for one crew, I call "SPS_PROC$GET_RESULTS(ACREW_ID, NULL)" (this is
only pseudo-code), "SPS_PROC$GET_RESULTS(NULL, NULL)" returns summary
for all data, "SPS_PROC$GET_RESULTS(ACREW_ID, ASALESREP_CODE)"
returns summary for particular salesrep in particular crew, etc...

MY QUESTION: Why SELECT statement

SELECT SUM(PRICE) FROM MY_DATA
WHERE ((:ACREW_ID IS NULL) OR (CREW_ID = :ACREW_ID)) AND
((:ASALESREP_CODE IS NULL) OR (SALESREP_CODE = :ASALESREP_CODE))

don't work alone? I always get compile error

ISC ERROR CODE:335544569
ISC ERROR MESSAGE:
Dynamic SQL Error
SQL error code = -804
Data type unknown

I'm using for IB access IB_SQL (powered by IBObjects) or EMS
Quickdesk (powered by FIBPlus) and FB RC2...
Statement like (:APARAM IS NULL) in SELECT WHERE clause is very
flexible, because in the case of n input parameters I don't have to
construct 2^n different SELECTs...

Thanks for your answer in advance and sorry for my english...

Tibor