MySQL rejects column with ON DELETE SET NULL but not ON DELETE CASCADE

问题

I have a simple MySQL database with some foreign keys on certain table columns. Some of the foreign key columns are set to ON DELETE SET NULL, and it seems to work fine. However, for one of the tables, I can't configure ON DELETE SET NULL - I can only configure ON DELETE CASCADE.

The initial tables look like this:

CREATE TABLE sessions(
session_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
name varchar(50) NOT NULL,
UNIQUE KEY uk_sessions(name))
COLLATE utf8_unicode_ci;

CREATE TABLE blocks(
block_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
name varchar(50) NOT NULL,
UNIQUE KEY uk_blocks(name))
COLLATE utf8_unicode_ci;

CREATE TABLE edot(
edah_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
name varchar(50) NOT NULL,
UNIQUE KEY uk_edot(name))
COLLATE utf8_unicode_ci;

CREATE TABLE campers(
camper_id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
edah_id int,
FOREIGN KEY fk_edah_id(edah_id) REFERENCES edot(edah_id)
ON DELETE SET NULL
ON UPDATE CASCADE,
session_id int,
FOREIGN KEY fk_session_id(session_id) REFERENCES sessions(session_id)
ON DELETE SET NULL
ON UPDATE CASCADE,
first varchar(50) NOT NULL,
last varchar(50) NOT NULL,
email varchar(50) NOT NULL,
needs_first_choice bool DEFAULT 0,
active bool NOT NULL DEFAULT 1)
COLLATE utf8_unicode_ci;

The above all works fine. The problem comes when I try to do this:

CREATE TABLE block_instances(
block_id int NOT NULL,
FOREIGN KEY fk_block_id(block_id) REFERENCES blocks(block_id)
ON DELETE CASCADE
ON UPDATE CASCADE,
session_id int,
FOREIGN KEY fk_session_id(session_id) REFERENCES sessions(session_id)
ON DELETE SET NULL
ON UPDATE CASCADE,
PRIMARY KEY pk_block_instances(block_id, session_id))
COLLATE utf8_unicode_ci;

This returns

ERROR 1005 (HY000): Can't create table 'chugbot_db.block_instances' (errno: 150)

The error goes away if I change the second ON DELETE SET NULL to ON DELETE CASCADE:

CREATE TABLE block_instances(
block_id int NOT NULL,
FOREIGN KEY fk_block_id(block_id) REFERENCES blocks(block_id)
ON DELETE CASCADE
ON UPDATE CASCADE,
session_id int,
FOREIGN KEY fk_session_id(session_id) REFERENCES sessions(session_id)
ON DELETE **CASCADE**
ON UPDATE CASCADE,
PRIMARY KEY pk_block_instances(block_id, session_id))
COLLATE utf8_unicode_ci;

works fine.

Can anyone tell me what is wrong with using ON DELETE SET NULL in the block_instances table here? It seems exactly the same as the ON DELETE SET NULL in the campers table.

回答1:

This turned out to be a bug in MySQL:

http://bugs.mysql.com/bug.php?id=80052

来源：https://stackoverflow.com/questions/34069835/mysql-rejects-column-with-on-delete-set-null-but-not-on-delete-cascade