SQL Join with NOT IN() does not work - mysql

Question

I have 2 tables that contain both the same key p_id:
test1 test2
+-------------+ +----------------------+
| p_id | name | | o_id | name | p_id |
+-------------+ +----------------------+
| 1 | Paul | | 1 | London | 1 |
| 2 | Marc | | 2 | Paris | 1 |
+-------------+ +----------------------+
Now I want to get all entries from test1 that have no relationship to test2.
In the example above I have abstracted my tables so RIGHT JOIN is not possible (in reality I have to join 4 tables).
SELECT a.*,b.*
FROM test1 a
LEFT JOIN test2 b
ON a.p_id=b.p_id
WHERE b.p_id NOT IN(SELECT DISTINCT p_id FROM test2);
I expect one row with p_id=2. However I get an empty result.
When I change my code into this:
SELECT a.*,b.*
FROM test1 a
LEFT JOIN test2 b
ON a.p_id=b.p_id
WHERE a.p_id NOT IN(SELECT DISTINCT p_id FROM test2);
Then it works fine. But why? I thought LEFT JOIN is processed first (1 row as result) and after that WHERE is processed (JOIN has not found p_id in test2so b.p_id is null - null is not in subselect - so still 1 row as result).
Could someone explain this behavior, please?

Answer 1

It has to do with how NULL is handled in comparisons.
To test/see, you can run simple queries like:
SELECT 1
FROM DUAL
WHERE NULL = NULL;
SELECT 1
FROM DUAL
WHERE NULL NOT IN (1, 2, 3);
Neither return a row because both conditions return NULL which is "not true".

Answer 2

As Uueerdo said, it's a NULL comparison issue. But that aside, you should really use an anti-join:
SELECT a.*,b.*
FROM test1 a
LEFT JOIN test2 b
ON a.p_id=b.p_id
WHERE b.p_id IS NULL;
It's cleaner and generally more efficient.

Answer 3

Nothing wrong with NOT IN, since you are filtering the right table in Where clause it is implicitly converted to INNER JOIN.
Without the Where clause result will be like this
+------+------+--------+--------+--------+
| p_id | name | o_id | name | p_id |
+------+------+--------+--------+--------+
| 1 | Paul | 1 | London | 1 |
| 1 | Paul | 2 | Paris | 1 |
| 2 | Marc | (null) | (null) | (null) |
+------+------+--------+--------+--------+
In this if you are applying the filter
WHERE b.p_id NOT IN(SELECT DISTINCT p_id FROM test2);
The sub-query returns 1 which is present in the last column of above result. So you are not getting any result.
In case if you are wondering why the last record having NULL is not returned since it is not 1. It is because NULL cannot compared using =, IN, NOT IN,etc.. We need to use IS operator for checking NULL
Proper way to do this would be using NOT EXISTS. Handles NULL values as well
select *
from test1 a
Where Not Exists (select 1 from test2 b Where a.p_id = b.p_id)

Answer 4

In general, WHERE is executed first, then JOIN. Also, when you use LEFT JOIN, it's the LEFT table that has everything included, so your shouldn't expect one row with p_id=2 after the JOIN, as you stated.