Why is this an issue?
WHERE clause condition that uses
NOT IN with a subquery will have unexpected results if that subquery returns
NULL. On the other hand,
NOT EXISTS subqueries work reliably under the same conditions.
This rule raises an issue when
NOT IN is used with a subquery. This rule doesn’t check if the selected column is a nullable column
because the rules engine has no information about the table definition. It’s up to the developer to review manually if the column is nullable.
Noncompliant code example
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
WHERE NOT EXISTS (SELECT 1 FROM another_table WHERE nullable_column = my_table.my_column)
WHERE my_column NOT IN (SELECT nullable_column FROM another_table WHERE nullable_column IS NOT NULL)