在很多时候操作数据库的时候，可能领导或DBA登陆了数据库，在执行update和delete时，忘记了加where，可能会导致清空表的悲剧，所以-U的好处就体现了。
1、mysql -U的帮助说明
-U, --safe-updates Only allow UPDATE and DELETE that uses keys. -U, --i-am-a-dummy Synonym for option --safe-updates, -U.
在mysql命令加上选项-U后，当发出没有WHERE或LIMIT关键字的UPDATE或DELETE时，mysql程序就会拒绝执行
2、指定-U登陆测试
[[email protected] ~]# mysql -uroot -p123 -S /data/3306/mysql.sock -U Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 14 Server version: 5.5.32-log MySQL Community Server (GPL) Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql> delete from erlianzhang.test; ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column mysql> quit Bye
不加条件就无法删除，这样就保护了数据，防止误操作。
3、防止领导或DBA误操作
[[email protected] ~]# alias mysql=‘mysql -U‘ [[email protected] ~]# mysql -uroot -p123 -S /data/3306/mysql.sock Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 15 Server version: 5.5.32-log MySQL Community Server (GPL) Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql> delete from erlianzhang.test; ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column mysql> delete from erlianzhang.test where Sno=5; Query OK, 1 row affected (0.02 sec) mysql> quit Bye [[email protected] ~]# echo "alias mysql=‘mysql -U‘" >>/etc/profile [[email protected] ~]# . /etc/profile [[email protected] ~]# tail -1 /etc/profile alias mysql=‘mysql -U‘
结论：在mysql 命令后加上-U参数后，当发出没有WHERE或LIMIT关键字的UPDATE或DELETE时，mysql程序拒绝执行，防止误操作给自己找麻烦。
转载至：http://blog.51cto.com/oldboy/1321061