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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