2019独角兽企业重金招聘Python工程师标准>>> 
由于建表时没注意导致数据库中有两张表的字符集变成了默认的 latin1，想把它转成utf8，于是使用以下命令：
mysql> ALTER TABLE logistics_express_trajectory CONVERT TO CHARACTER SET utf8_general_ci;
ERROR 1115 (42000): Unknown character set: 'utf8_general_ci'
转换失败，推想可能是表中有记录不允许转换，于是想重建表并指定字符集为utf8，但是由于是生产库，表里面的记录不能直接drop，于是先把数据库表结构及记录导出：
mysqldump -u logistics -p logistics logistics_express_trajectory > /home/logistics/logistics_express_trajectory.sql
然后再输入密码即可导出，接着执行drop语句：
mysql> drop table logistics_express_trajectory;
ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails
失败，原因大概是因为这个表与其它表有外建约束，于是想直接用delete语句把记录清空再改字符集吧，于是：
mysql> delete from logistics_express_trajectory;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`logistics`.`logistics_order_trajectory`, CONSTRAINT `Reflogistics_express_trajectory11` FOREIGN KEY (`trajectory_id`) REFERENCES `logistics_express_trajectory` (`id`))
还是失败，还是因为表被其它表关联，不能删除记录。根据外键名称找到对应的表：
use INFORMATION_SCHEMA;
select table_name, column_name from KEY_COLUMN_USAGE where CONSTRAINT_NAME = 'Reflogistics_express_trajectory11';
+----------------------------+---------------+
| table_name | column_name |
+----------------------------+---------------+
| logistics_order_trajectory | trajectory_id |
+----------------------------+---------------+
表名为logistics_order_trajectory，正好也要修改改该表的字符集，于是干脆也导出该表的记录并重建：
mysqldump -u logistics -p logistics logistics_order_trajectory > /home/logistics/logistics_order_trajectory.sql
然后再输入密码即可导出，接着按顺序执行drop语句：
mysql> use logistics;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> drop table logistics_order_trajectory;
Query OK, 0 rows affected (0.04 sec)
mysql> drop table logistics_express_trajectory;
Query OK, 0 rows affected (0.04 sec)
两张表都drop成功，然后再修改导出文件中建表的字符集为utf8，再按顺序导入两张表的结构及记录即可（被依赖的表要先导噢）
source /home/logistics/logistics_express_trajectory.sql;
source /home/logistics/logistics_order_trajectory.sql;
ok,成功，再使用语句查看两张表的字符集看是否修改过来了：
mysql> SELECT table_name, table_type, engine,TABLE_COLLATION FROM information_schema.tables where table_name='logistics_express_trajectory' or table_name='logistics_order_trajectory';
+------------------------------+------------+--------+-----------------+
| table_name | table_type | engine | TABLE_COLLATION |
+------------------------------+------------+--------+-----------------+
| logistics_express_trajectory | BASE TABLE | InnoDB | utf8_general_ci |
| logistics_order_trajectory | BASE TABLE | InnoDB | utf8_general_ci |
+------------------------------+------------+--------+-----------------+
已经修改过来了，接着再补上外键约束：
ALTER TABLE logistics_express_trajectory ADD CONSTRAINT Reflogistics_member_express7
FOREIGN KEY (member_express_id)
REFERENCES logistics_member_express(id)
;
mysql> ALTER TABLE logistics_order ADD CONSTRAINT Reflogistics_member_express8
-> FOREIGN KEY (member_express_id)
-> REFERENCES logistics_member_express(id)
-> ;
ERROR 1005 (HY000): Can't create table 'logistics.#sql-d971_11498' (errno: 121)
mysql> ALTER TABLE logistics_order_trajectory ADD CONSTRAINT Reflogistics_express_trajectory11
-> FOREIGN KEY (trajectory_id)
-> REFERENCES logistics_express_trajectory(id)
-> ;
ERROR 1005 (HY000): Can't create table 'logistics.#sql-d971_11498' (errno: 121)
擦，执行失败！下次再解决吧，出去陪老婆了！
好了，回来了，继续解决吧。
因为logistics_order和logistics_order_trajectory表虽然被drop掉了，但并没有把之前建表时建立的外键约束删除掉，所以怀疑是不是因为外键名称重名冲突了，于是用以下语句根据约束名查找看是否有对应的记录存在：
use INFORMATION_SCHEMA;
mysql> select table_name, column_name from KEY_COLUMN_USAGE where CONSTRAINT_NAME = 'Reflogistics_member_express8';
+-----------------+-------------------+
| table_name | column_name |
+-----------------+-------------------+
| logistics_order | member_express_id |
+-----------------+-------------------+
1 row in set (0.01 sec)
mysql> select table_name, column_name from KEY_COLUMN_USAGE where CONSTRAINT_NAME = 'Reflogistics_express_trajectory11';
+----------------------------+---------------+
| table_name | column_name |
+----------------------------+---------------+
| logistics_order_trajectory | trajectory_id |
+----------------------------+---------------+
1 row in set (0.01 sec)
果然都存在，drop掉再重建就好了
mysql> ALTER TABLE logistics_order DROP FOREIGN KEY Reflogistics_member_express8;
Query OK, 8 rows affected (0.27 sec)
Records: 8 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE logistics_order ADD CONSTRAINT Reflogistics_member_express8 FOREIGN KEY (member_express_id) REFERENCES logistics_member_express(id);
Query OK, 8 rows affected (0.27 sec)
Records: 8 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE logistics_order_trajectory DROP FOREIGN KEY Reflogistics_express_trajectory11;
Query OK, 5 rows affected (0.26 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE logistics_order_trajectory ADD CONSTRAINT Reflogistics_express_trajectory11 FOREIGN KEY (trajectory_id) REFERENCES logistics_express_trajectory(id);
Query OK, 5 rows affected (0.26 sec)
Records: 5 Duplicates: 0 Warnings: 0
OK,可以了，全部解决！