1. 测试数据
mysql> select* from exam1;
+------+-----------+---------+------+---------+
| id | name | Chinese | Math | English |
+------+-----------+---------+------+---------+
| 1 | 唐三藏 | 67.0 | 98.0 | 56.0 |
| 2 | 孙悟空 | 87.0 | 78.0 | 77.0 |
| 3 | 猪悟能 | 88.0 | 98.0 | 90.0 |
| 4 | 曹孟德 | 82.0 | 84.0 | 67.0 |
| 5 | 刘玄德 | 55.0 | 85.0 | 45.0 |
| 6 | 孙权 | 70.0 | 73.0 | 78.0 |
| 7 | 宋公明 | 75.0 | 65.0 | 30.0 |
| NULL | NULL | NULL | NULL | 90.0 |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)2. Update 修改
update操作，是真正在改硬盘了；这样的修改是"持久有效" 的，所以一定要确保 update 的修改是改对的；
update 表名 set 列名 = 值,列名 = 值.....;语法
UPDATE [LOW_PRIORITY] [IGNORE] table_referenceSET assignment[，assignment]... [WHERE where_condition][ORDER BY ...][LIMIT row_count]-- set 在这里的意思是设置示例
更新操作
一次修改一个列
把孙悟空数学成绩修改成 80
update exam1 set Math = 80 where name = '孙悟空';Query OK, 1 row affected (0.06 sec)Rows matched: 1 Changed: 1 Warnings: 0 -- 匹配和修改一行数据把所有数学成绩在原有基础上加10分
update exam1 set Math = Math + 10 where id is not null;ERROR 1264 (22003): Out of range value for column 'Math' at row 1-- 超出 decimal(3,1)的范围，此时是真正修改硬盘数据，所以修改失败把所有数学成绩在原有基础上减10分
update exam1 set Math = Math - 10 ;Query OK, 7 rows affected (0.01 sec)Rows matched: 8 Changed: 7 Warnings: 0-- 匹配的行数和修改的行数不同，说明如果修改的数据基础是 null，则无法修改一次修改多个列
把曹孟德的语文成绩修改为60，数学成绩修改为70
update exam1 set Chinese = 60 , Math = 70 where name = '曹孟德';Query OK, 1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0引入 order by 操作
将总成绩倒数前三的3位同学的数学成绩加上20分
-- 查看总分倒数前三的同学select* from exam1 where Chinese + Math + English is not null order by Chinese + Math + English limit 3;-- 更新总分倒数前三的同学的数学成绩update exam1 set Math = Math + 20 where Chinese + Math + English is not null order by Chinese + Math + English limit 3;特殊情况：好几个同学成绩相同，倒数前三的成绩里有不止3个同学
Update 注意事项
3. Delete 删除
语法
DELETE FROM tbl_name [WHERE where_condition] [ORDER BY ...] [LIMIT row_count]delete from 表名 where 条件/ order by / limit;示例
删除操作
删除孙悟空同学的考试成绩
delete from exam1 where name = '孙悟空';Query OK, 1 row affected (0.01 sec)Delete 注意事项
