== How to fix it

Use `NOT EXISTS` or `IS NOT NULL` instead of `NOT IN` when the subquery may return `NULL` values.

=== Code examples

==== Noncompliant code example

[source,sql]
----
SELECT *
FROM my_table
WHERE my_column NOT IN (SELECT nullable_column FROM another_table)  -- Noncompliant; "nullable_column" may contain 'NULL' value and the whole SELECT query will return nothing
----

==== Compliant solution

[source,sql]
----
SELECT *
FROM my_table
WHERE NOT EXISTS (SELECT 1 FROM another_table WHERE nullable_column = my_table.my_column)
----

[source,sql]
----
SELECT *
FROM my_table
WHERE my_column NOT IN (SELECT nullable_column FROM another_table WHERE nullable_column IS NOT NULL)
----