Union and NULL

[Peter Heiberg]

I have noticed a strange behaviour when using union. Anyone who knows why this is happening? Is it a bug or a feature?

This is my problem when running the below queries separately without a union I get the following results:

+-----------+-------------+
| folder_id | folder_childid |
+-----------+-------------+
| 1 | 2 |
+-----------+-------------+

+-----------+-------------+
| folder_id | folder_childid |
+-----------+-------------+
| 2 | [NULL] |
| 3 | [NULL] |
+-----------+-------------+
The query:

SELECT folder_parentid AS folder_id, folder_id AS folder_childid
FROM folder
WHERE folder_parentid IS NOT NULL
UNION ALL
SELECT folder_id, null AS folder_childid
FROM folder
WHERE folder_parentid IS NULL

But when I use a UNION ALL on the queries, the NULLs are replaced by 0!

+-----------+-------------+
|__folder_id_|_folder_childid|
+-----------+-------------+
|______1_|________2_|
|______2_|________0_|
|______3_|________0_|
+-----------+-------------+

Why is this?