IN operator caluse having a NULL value seems to work - mysql

Question

I have an 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 |
+--------+----------+-----------+------------+----------+-----------------+---------+--------------------+--------------------+
If I do
SELECT emp_id, fname, lname, title
FROM employee
WHERE emp_id IN (
SELECT superior_emp_id
FROM employee
);
I get:
+--------+---------+-----------+--------------------+
| 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 |
+--------+---------+-----------+--------------------+
There is a NULL value in the superior_emp_id column.
If the IN operator is equivalent to field=val1 OR field=val2 OR field=val3 OR field=null why does this query not fail or give some error?

Answer 1

It works just fine. You may be confusing in and not in.
If you run not in, then you will get no results. Why? Well, consider 1 not in (2, 3). That evaluates to true. No-brainer.
Then 2 not in (2, 3). That evaluates to false. Just as it should.
But . . . what about these two:
2 not in (2, 3, NULL)
1 not in (2, 3, NULL)
The first is false, because "2" is indeed in the list. The second is . . . well, NULL is not a value. It means "unknown". So, it could be "1" or something else. Hence, it evaluates to NULL. And NULL is treated the same as false in a where.
You can work it out, but this does not occur with in. For instance, in your case, NULL or 1=1 evaluates to true.
For this reason, I strongly recommend always using not exists rather than not in with a subquery. A corollary is that I recommend exists rather than in, but just so the habit of using exists is engrained.

Answer 2

x=NULL predicate evaluates to UNKNOWN
With regards to the OR operator:
TRUE OR UNKNOWN = TRUE
FALSE OR UNKNOWN = UNKNOWN (which is treated as a FALSE in the WHERE clause)
see: https://en.wikipedia.org/wiki/Three-valued_logic