In db2, null could be anything. But some time when we join tables by using join condition which involves null values, we should remember that :
a.cola = b.colb -- if both are null, the condition is false, that is null = null returns false.
a.cola <> b.colb -- if both are null, the condition is false, that is NULL <> NULL returns False
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7 = NULL => False
7 > NULL => False
7 <> NULL => False
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NULL = NULL => False
NULL <> NULL => False
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NULL IN( 1, NULL, 3) => False
NULL IN (1, 3) => False
1 IN (1, NULL, 3) => True
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