我得到 errno 150 并且不知道为什么。我读过一百万个其他答案说你必须在父列上设置一个索引，我这样做我不确定为什么我仍然会收到这个错误。
父表索引：
mysql> show indexes from plans;
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| plans | 0 | PRIMARY | 1 | plan_id | A | 86 | NULL | NULL | | BTREE | | |
| plans | 0 | plan_id_UNIQUE | 1 | plan_id | A | 86 | NULL | NULL | | BTREE | | |
| plans | 1 | plan_id | 1 | plan_id | A | 86 | NULL | NULL | | BTREE | | |
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.08 sec)
子表索引：
mysql> show indexes from promos;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| promos | 0 | PRIMARY | 1 | promo_id | A | 0 | NULL | NULL | | BTREE | | |
| promos | 1 | plan_id | 1 | plan_id | A | 0 | NULL | NULL | | BTREE | | |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.06 sec)
错误：
mysql> alter table promos add foreign key (plan_id) references plans (plan_id) on delete cascade on update cascade;
ERROR 1005 (HY000): Can't create table 'dev.#sql-2ce_599a' (errno: 150)