While ``++NOT IN++`` can be far more efficient than ``++NOT EXISTS++`` in a query, it may return misleading results if the column in question contains null values.
== Noncompliant Code Example
[source,text]
----
SELECT COUNT(*) FROM emp
WHERE empno NOT IN ( SELECT mgr FROM emp );
== Compliant Solution
SELECT COUNT(*) FROM emp T1
WHERE NOT EXISTS ( SELECT NULL FROM emp T2 WHERE t2.mgr = t1.empno );
