SQL IN () is including NULL values - mysql

Question

Employee table:
+--------+----------+-----------+------------+----------+-----------------+---------+--------------------+--------------------+
| emp_id | fname | lname | start_date | end_date | superior_emp_id | dept_id | title | assigned_branch_id |
+--------+----------+-----------+------------+----------+-----------------+---------+--------------------+--------------------+
| 1 | Michael | Smith | 2005-06-22 | NULL | NULL | 3 | President | 1 |
| 2 | Susan | Barker | 2006-09-12 | NULL | 1 | 3 | Vice President | 1 |
| 3 | Robert | Tyler | 2005-02-09 | NULL | 1 | 3 | Treasurer | 1 |
| 4 | Susan | Hawthorne | 2006-04-24 | NULL | 3 | 1 | Operations Manager | 1 |
| 5 | John | Gooding | 2007-11-14 | NULL | 4 | 2 | Loan Manager | 1 |
| 6 | Helen | Fleming | 2008-03-17 | NULL | 4 | 1 | Head Teller | 1 |
| 7 | Chris | Tucker | 2008-09-15 | NULL | 6 | 1 | Teller | 1 |
| 8 | Sarah | Parker | 2006-12-02 | NULL | 6 | 1 | Teller | 1 |
| 9 | Jane | Grossman | 2006-05-03 | NULL | 6 | 1 | Teller | 1 |
| 10 | Paula | Roberts | 2006-07-27 | NULL | 4 | 1 | Head Teller | 2 |
| 11 | Thomas | Ziegler | 2004-10-23 | NULL | 10 | 1 | Teller | 2 |
| 12 | Samantha | Jameson | 2007-01-08 | NULL | 10 | 1 | Teller | 2 |
| 13 | John | Blake | 2004-05-11 | NULL | 4 | 1 | Head Teller | 3 |
| 14 | Cindy | Mason | 2006-08-09 | NULL | 13 | 1 | Teller | 3 |
| 15 | Frank | Portman | 2007-04-01 | NULL | 13 | 1 | Teller | 3 |
| 16 | Theresa | Markham | 2005-03-15 | NULL | 4 | 1 | Head Teller | 4 |
| 17 | Beth | Fowler | 2006-06-29 | NULL | 16 | 1 | Teller | 4 |
| 18 | Rick | Tulman | 2006-12-12 | NULL | 16 | 1 | Teller | 4 |
+--------+----------+-----------+------------+----------+-----------------+---------+--------------------+--------------------+
Query:
SELECT emp_id, fname, lname, title
FROM employee
WHERE emp_id IN (SELECT superior_emp_id FROM employee);
Query Result:
+--------+---------+-----------+--------------------+
| emp_id | fname | lname | title |
+--------+---------+-----------+--------------------+
| 1 | Michael | Smith | President |
| 3 | Robert | Tyler | Treasurer |
| 4 | Susan | Hawthorne | Operations Manager |
| 6 | Helen | Fleming | Head Teller |
| 10 | Paula | Roberts | Head Teller |
| 13 | John | Blake | Head Teller |
| 16 | Theresa | Markham | Head Teller |
+--------+---------+-----------+--------------------+
Subquery result:
+-----------------+
| superior_emp_id |
+-----------------+
| NULL |
| 1 |
| 1 |
| 3 |
| 4 |
| 4 |
| 4 |
| 4 |
| 4 |
| 6 |
| 6 |
| 6 |
| 10 |
| 10 |
| 13 |
| 13 |
| 16 |
| 16 |
+-----------------+
If the subquery SELECT superior_emp_id FROM employee returns NULL for Michael Smith how is it that the IN() operator returns it in the final result set? I thought nothing was equal to null.

Answer 1

If the subquery SELECT superior_emp_id FROM employee returns NULL for Michael Smith how is it that the IN() operator returns it in the final result set?
Short answer, it doesn't.
The subquery effectively returns the whole set of superior_emp_ids [NULL, 1, 1, 3, 3, 4, 4, 6, 6, 6, 4, 10, 10, 4, 13, 13, 4, 16, 16] for each row.
Your WHERE clause tests each emp_id to see if it is IN this set. And IN is basically a series of equals comparisons OR'd together.
Michael's emp_id is 1 and his row is returned because 1 = NULL OR 1 = 1 .... which can be written as FALSE OR TRUE .... returns TRUE.
You are correct in assuming that NULL doesn't equal anything, including NULL, so WHERE NULL IN (NULL, 1, FALSE, ... anything you like ...) will return FALSE. But that is not what's happening in your example.
N.B. To avoid any confusion it is much better to avoid NULL records on either side of an IN clause where possible as referenced by #Donal

Answer 2

SELECT superior_emp_id FROM employee
returns [NULL, 1, 3, 4, 6, 10, 13, 16]. I do not see the problem here.

Answer 3

Have a look at the ANSI_NULLS setting in SQL Server.
Transact-SQL supports an extension that allows for the comparison
operators to return TRUE or FALSE when comparing against null values.
This option is activated by setting ANSI_NULLS OFF. When ANSI_NULLS is
OFF, comparisons such as ColumnA = NULL return TRUE when ColumnA
contains a null value and FALSE when ColumnA contains some value
besides NULL.
Taken from here.
If you don't want the NULL value you will need to add a WHERE clause to the sub query. For example:
SELECT emp_id, fname, lname, title
FROM employee
WHERE emp_id IN (SELECT superior_emp_id FROM employee WHERE superior_emp_id IS NOT NULL);