SQL parser bug?

Question

Is there a known problem with NOT IN operator? The following statement returns "no rows selected" message
SELECT col_a FROM tab_a WHERE col_a NOT IN (
SELECT col_b FROM tab_b);
while the following two statements both return the expected result
SELECT col_a FROM tab_a MINUS SELECT col_b FROM tab_b;
SELECT col_a FROM tab_a WHERE NOT EXISTS (
SELECT * FROM tab_b WHERE col_b = tab_a.col_a);
Note 1: This failure only happens on certain tables, but not on other tables in the same database.
Note 2: both of tab_a.col_a and tab_b.col_b are VARCHAR2(32).
Note 3: Oracle8.1.7 on Tru64.
Any insights are appreciated.
Robin

Answer 1

My first instinct is that it has to do with handling nulls. Each of those statements will do it slightly differently.
col_a NOT IN () : if col_a is null won't return row
MINUS : Oracle will correctly compare nulls
NOT EXISTS ( = col_a) : if col_a is null won't return row so 'not exists' will be true and master query row will always return row (even if they were both null).
Have you checked that?
Richard