An Interesting Feature of NOT IN and Multi-Row Subqueries

Take the following simple SQL statement:

SELECT *
FROM dual
WHERE 'x' NOT IN
(SELECT 'a' FROM dual);

Since ‘x’ cannot be found in our subquery, you’d expect this to return a row from Dual right? Indeed it does:

SQL> SELECT *
  2  FROM dual
  3  WHERE 'x' NOT IN
  4  (SELECT 'a' FROM dual);

D
-
X

What about in the following case?

SELECT *
FROM dual
WHERE 'x' NOT IN
(SELECT 'a' FROM dual
 UNION ALL
 SELECT NULL FROM dual);

You might expect this to return a row as well since ‘x’ is still not in our subquery (which now returns two values: ‘x’ and NULL).

You’d be wrong, however. This query will in fact return no rows:

SQL> SELECT *
  2  FROM dual
  3  WHERE 'x' NOT IN
  4  (SELECT 'a' FROM dual
  5   UNION ALL
  6   SELECT NULL FROM dual);

no rows selected

If the subquery referenced by your NOT IN statement contains any NULL values, the NOT IN condition will evaluate to unknown.

It’s worth noting that this is not the case if you use an IN as opposed to a NOT IN:

SQL>  SELECT *
  2   FROM dual
  3   WHERE 'a' IN
  4   (SELECT 'a' FROM dual
  5    UNION ALL
  6    SELECT NULL FROM dual);

D
-
X

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