== Why is this an issue?

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

[source,text]
----
SELECT COUNT(*) FROM emp T1
   WHERE NOT EXISTS ( SELECT NULL FROM emp T2 WHERE t2.mgr = t1.empno );
----