Question

select 'true' from dual where 1 not in (null,1);

when we execute this which will result nothing

what my question is:

is the above query is logically equivalent to

select 'true' from dual where 1 != null and 1 != 1;

which will result nothing just as above statement

Please clarify?

Answer 1

Correct (but note that IN is an operator, not a clause and it works like this in SQL in general, not only for Oracle).

where 1 not in (null,1)

is equivalent to:

where 1 != null and 1 != 1

which should really be written as:

WHERE 1 NOT IN (NULL, 1)

and

WHERE 1 <> NULL AND 1 <> 1

which is the same as:

WHERE (1 <> NULL) AND (1 <> 1)

which evaluates to:

WHERE UNKNOWN AND FALSE

and further as:

WHERE FALSE

So, it correctly returns no rows.

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Notice that if you had WHERE 1 NOT IN (NULL, 2), it would evaluate to WHERE UNKNOWN (left as an exercise) and no rows would be returned either.