最近修改mysql数据库表中的字段长度时报错，执行更改的sql语句：
ALTER TABLE server_list MODIFY COLUMN server_lip CHAR(25);

报错信息：
1 queries executed, 0 success, 1 errors, 0 warnings
查询：alter table server_list modify column server_lip char(25)
错误代码： 1833
Cannot change column 'server_lip': used in a foreign key constraint 'server_history_ibfk_1' of table 'OMServer.server_history'

原因：server_list表中的server_lip字段为server_history表的外键，server_history建表语句：
CREATE TABLE server_history (
ID int(11) NOT NULL AUTO_INCREMENT COMMENT '主键ID',
history_id int(11) NOT NULL COMMENT '事件ID',
history_ip char(15) NOT NULL COMMENT '事件IP地址',
history_user char(15) NOT NULL COMMENT '事件用户名',
history_datetime datetime NOT NULL COMMENT '事件时间',
db_datetime timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入库时间',
history_command char(255) NOT NULL COMMENT '事件命令',
PRIMARY KEY (ID),
KEY history_ip (history_ip),
CONSTRAINT server_history_ibfk_1 FOREIGN KEY (history_ip) REFERENCES server_list (server_lip)
) ENGINE=InnoDB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8 COMMENT='操作事件表'

解决步骤：
1.锁表
LOCK TABLES server_history WRITE,server_list WRITE;
2.查看是否被锁

3.解除server_history表外键约束
ALTER TABLE server_history DROP FOREIGN KEY server_history_ibfk_1;
4.更改server_list表server_lip字段长度
ALTER TABLE server_list MODIFY COLUMN server_lip CHAR(25);
查看表结构发现已完成字段长度更新

5.新增server_history表外键约束
ALTER TABLE server_history ADD CONSTRAINT server_history_ibfk_1 FOREIGN KEY (history_ip) REFERENCES server_list (server_lip);
6.解除锁定
UNLOCK TABLES;