mysql> alter table gradeinfo change s_num num int(10);
ERROR 1025 (HY000): Error on rename of '.\example\#sql-788_4' to '.\example\gradeinfo' (errno: 150)

网上查出的原因:

 

  • 真实的原因及解决办法:
show index 发现有和外键同名的索引存在
mysql> show index from CORE_FUNCTION;
+---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| CORE_FUNCTION | 0 | PRIMARY | 1 | FUNCTION_ID | A | 0 | NULL | NULL | | BTREE | |
| CORE_FUNCTION | 1 | FKDB8410785D1C928B | 1 | PARENT | A | 0 | NULL | NULL | YES | BTREE | |
+---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.01 sec)
删除掉索引
mysql> alter table CORE_FUNCTION drop index FKDB8410785D1C928B;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
再show keys看,外键不见了
mysql> show keys from CORE_FUNCTION;
+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| CORE_FUNCTION | 0 | PRIMARY | 1 | FUNCTION_ID | A | 0 | NULL | NULL | | BTREE | |
+---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
1 row in set (0.00 sec)

显然这是MYSQL的一个BUG:


1.MYSQL在建外键后,会自动建一个同名的索引


2.而

删除外键的时候,这个同名索引如果没被删,则MYSQL认为外键