mysql - 更改 SQL 表上的 ondelete 行为

Question

我以前做过:

ALTER TABLE `cms__model__file_taggable_tag` ADD CONSTRAINT 
cms__model__file_taggable_tag_id_cms__model__file_id FOREIGN KEY (id) REFERENCES 
cms__model__file (id), ADD CONSTRAINT 
cms__model__file_taggable_tag_tag_id_taggable_tag_id FOREIGN KEY (tag_id) 
REFERENCES taggable_tag (id) ON DELETE CASCADE ON UPDATE CASCADE; 

但我打算执行(忘记添加更新级联约束):

ALTER TABLE `cms__model__file_taggable_tag` ADD CONSTRAINT 
cms__model__file_taggable_tag_id_cms__model__file_id FOREIGN KEY (id) REFERENCES 
cms__model__file (id) ON DELETE CASCADE ON UPDATE CASCADE, ADD CONSTRAINT 
cms__model__file_taggable_tag_tag_id_taggable_tag_id FOREIGN KEY (tag_id) 
REFERENCES taggable_tag (id) ON DELETE CASCADE ON UPDATE CASCADE; 

我怎样才能删除以前的约束并应用正确的约束？

---

尝试过:

ALTER TABLE `cms__model__file_taggable_tag` DROP CONSTRAINT 
cms__model__file_taggable_tag_id_cms__model__file_id

得到:

#1064 - You have an error in your SQL syntax; check the manual that corresponds
to your MySQL server version for the right syntax to use near 'CONSTRAINT 
cms__model__file_taggable_tag_id_cms__model__file_id' at line 1

Answer 1

使用 SHOW CREATE TABLE tableName 检查 MySQL 是否实际创建了约束以及它是否更改(截断)了它的名称。

然后使用DROP FOREIGN KEY，而不是DROP CONSTRAINT:

ALTER TABLE cms__model__file_taggable_tag
  DROP FOREIGN KEY cms__model__file_taggable_tag_id_cms__model__file_id

查看 MySQL 文档: ALTER TABLE 和 FOREIGN KEY Constraints