Some more weird NULL behavior

Hi:

I tried the following:
CREATE TABLE x(
a NUMERIC(5,0),
b VARCHAR(5)
);

CREATE TABLE y(
a INTEGER,
b VARCHAR(5)
);

INSERT INTO x(b) VALUES ('LUDZ');
INSERT INTO y(b) VALUES ('TEST');

SELECT x.b,y.b FROM x,y WHERE x.a=y.a
returns zero rows.

However:
SELECT b FROM x WHERE a IS NULL returns 'LUDZ'
SELECT b from y WHERE a IS NULL returns 'TEST'

I also tried
SELECT x.b,y.b
FROM x,y
WHERE x.a = CAST(y.a AS INTEGER);

But it also returns zero rows.

Thank you in advance,
ludwig.

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