MySQL拒绝使用ON DELETE SET NULL而不是ON DELETE CASCADE的列

怡情水云间

我有一个简单的MySQL在某些表列中，数据库中有一些外键。一些外键列被设置为ON DELETE SET
NULL，而且看起来工作正常。然而，我无法配置其中一张表。ON DELETE SET NULL-我只能配置ON DELETE CASCADE。
初始表如下：
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 NULLON UPDATE CASCADE,session_id int,FOREIGN KEY fk_session_id(session_id) REFERENCES sessions(session_id)ON DELETE SET NULLON 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;所有这些工作都很正常。当我试图这样做时，问题来了：
CREATE TABLE block_instances(block_id int NOT NULL,FOREIGN KEY fk_block_id(block_id) REFERENCES blocks(block_id)ON DELETE CASCADEON UPDATE CASCADE,session_id int,FOREIGN KEY fk_session_id(session_id) REFERENCES sessions(session_id)ON DELETE SET NULLON UPDATE CASCADE,PRIMARY KEY pk_block_instances(block_id,session_id))COLLATE utf8_unicode_ci;这返回
ERROR 1005 (HY000): Can't create table 'chugbot_db.block_instances' (errno: 150)假如第二个ON DELETE SET NULL更改为ON DELETE CASCADE，错误消失：
) C9 v* [2 J$ X) z; I3 f1 ]9 e: hCREATE TABLE block_instances(block_id int NOT NULL,FOREIGN KEY fk_block_id(block_id) REFERENCES blocks(block_id)ON DELETE CASCADEON 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;工作正常。
* Y1 ~2 T  s6 p# m+ y, b谁能告诉我在这里block_instances表中使用ON DELETE SET NULL有什么问题吗？campers表中的ON DELETE
$ a0 l9 M1 M* }) VSET NULL完全相同。
                                                               
    解决方案: