General error;-8 POS(1) Execution failed, parse again

[igo igo]

Hi, I have problem with this select:

SELECT customers.ID as c_ID, messages.ID as m_ID, true
FROM customers customers, messages messages
WHERE ((customers.ID, messages.ID) IN (SELECT customer_id, message_id FROM used_messages))
UNION
SELECT customers.ID as c_ID, messages.ID as m_ID, false
FROM customers, messages
WHERE ((customers.ID, messages.ID) NOT IN (SELECT customer_id, message_id FROM used_messages))

I get "General error;-8 POS(1) Execution failed, parse again."
What do I have to do?
I use MaxDB 7.5.0

Tables:


CREATE TABLE customers (
id SERIAL NOT NULL,
name VARCHAR(20) NOT NULL,
PRIMARY KEY (id)
);

CREATE TABLE messages (
id SERIAL NOT NULL,
name VARCHAR(20) NOT NULL,
PRIMARY KEY (id)
);

CREATE TABLE used_messages (
customer_id INTEGER NOT NULL,
message_id INTEGER NOT NULL,
PRIMARY KEY (customer_id, message_id)
);

ALTER TABLE used_messages
ADD FOREIGN KEY (message_id)
REFERENCES messages ;


ALTER TABLE used_messages
ADD FOREIGN KEY (customer_id)
REFERENCES customers ;