『 MySQL数据库 』CRUD之UD,表的数据更新(修改)及删除

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Update (更新/修改)

该语法主要针对于对数据库的表内数据进行修改;
一般该操作需要配合条件筛选,否则操作将影响整张表;
语法:

UPDATE table_name SET column = expr[,column = expr ... ]
[WHERE ...] [ORDER BY ...] [LIMIT ...]

存在一张表:

+----+---------+---------+------+---------+
| id | name    | chinese | math | english |
+----+---------+---------+------+---------+
|  1 | Lihua   |      67 |   88 |      90 |
|  2 | Liming  |      57 |   58 |      70 |
|  3 | Zhaolao |      66 |   80 |      47 |
|  4 | Wu      |      76 |   70 |      47 |
|  5 | Wuqi    |      88 |   43 |      80 |
|  6 | Liqiang |      89 |   92 |      90 |
|  7 | Qinsu   |      90 |   74 |      67 |
|  8 | Zhaoli  |      54 |   74 |     100 |
+----+---------+---------+------+---------+

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修改单行数据的某个字段内的数据

示例:将name字段为Lihua的chinese字段数据修改为100

mysql> select name,chinese from Point where name ='Lihua';# 打印数据进行观察
+-------+---------+
| name  | chinese |
+-------+---------+
| Lihua |      67 |
+-------+---------+
1 row in set (0.00 sec)

mysql> update Point set chinese=100 where name='Lihua';# 修改数据
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select name,chinese from Point where name ='Lihua';# 打印数据进行观察
+-------+---------+
| name  | chinese |
+-------+---------+
| Lihua |     100 |
+-------+---------+
1 row in set (0.00 sec)

也可对单行数据的多个字段数据进行修改,前提是这个数据具有唯一属性(体现了主键约束的重要性);

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配合LIMIT分页与ORDER BY 对符合条件的多条数据进行修改

示例:将字段条件chinese+math+english前3大的数据中的math字段数据+30

# 打印数据进行查看 -- 修改前
mysql> select name,math from Point order by math+chinese+english desc limit 3;
+---------+------+
| name    | math |
+---------+------+
| Lihua   |   88 |
| Liqiang |   92 |
| Qinsu   |   74 |
+---------+------+
3 rows in set (0.00 sec)

mysql> select name,math+chinese+english from Point order by math+chinese+english desc limit 3;
+---------+----------------------+
| name    | math+chinese+english |
+---------+----------------------+
| Lihua   |                  278 |
| Liqiang |                  271 |
| Qinsu   |                  231 |
+---------+----------------------+
3 rows in set (0.00 sec)

# 修改数据
mysql> update Point set math=math+30 order by math+chinese+english desc limit 3;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

# 打印数据进行查看 -- 修改后
mysql> select name,math+chinese+english from Point order by math+chinese+english desc limit 3;
+---------+----------------------+
| name    | math+chinese+english |
+---------+----------------------+
| Lihua   |                  308 |
| Liqiang |                  301 |
| Qinsu   |                  261 |
+---------+----------------------+
3 rows in set (0.00 sec)

mysql> select name,math from Point order by math+chinese+english desc limit 3;
+---------+------+
| name    | math |
+---------+------+
| Lihua   |  118 |
| Liqiang |  122 |
| Qinsu   |  104 |
+---------+------+

当需要修改的数据条件繁琐的前提可以先使用SELECT FROM语句先对需要修改的数据进行查看筛选,再对该SELECT FROM语句根据筛选条件修改为对应的UPDATE SET语句对数据进行修改!

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对整表的某个数据字段进行修改

在UPDATE SET语句中若是没有用WHERE语句进行筛选则更新整表!
在SQL中对整表的操作需要慎重;
示例:将整表中的english字段数据修改为 *=2 ;

# 使用SELECT FROM语句对数据进行查看 -- 修改前
mysql> select id,name,english from Point;
+----+---------+---------+
| id | name    | english |
+----+---------+---------+
|  1 | Lihua   |      90 |
|  2 | Liming  |      70 |
|  3 | Zhaolao |      47 |
|  4 | Wu      |      47 |
|  5 | Wuqi    |      80 |
|  6 | Liqiang |      90 |
|  7 | Qinsu   |      67 |
|  8 | Zhaoli  |     100 |
+----+---------+---------+
8 rows in set (0.00 sec)

# 对数据进行修改
mysql> update Point set english*=2 ; #SQL语句中不支持*=,+=等语法!
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '*=2' at line 1# error

mysql> update Point set english=english*2 ;# 正确
Query OK, 8 rows affected (0.01 sec)
Rows matched: 8  Changed: 8  Warnings: 0

# 使用SELECT FROM语句对数据进行查看 -- 修改后
mysql> select id,name,english from Point;
+----+---------+---------+
| id | name    | english |
+----+---------+---------+
|  1 | Lihua   |     180 |
|  2 | Liming  |     140 |
|  3 | Zhaolao |      94 |
|  4 | Wu      |      94 |
|  5 | Wuqi    |     160 |
|  6 | Liqiang |     180 |
|  7 | Qinsu   |     134 |
|  8 | Zhaoli  |     200 |
+----+---------+---------+
8 rows in set (0.00 sec)

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Delete (删除数据)

该语句一般用于对某条数据的删除,也可以删除整表的数据;
语法:

DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...]

存在一张表(to_del):

+-----+-----------+
| id  | name      |
+-----+-----------+
| 001 | Lihua     |
| 002 | Liming    |
| 003 | Zhangchao |
| 004 | Zhaoli    |
| 005 | Lili      |
+-----+-----------+

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删除单条数据

示例:删除id字段为2的数据

mysql> select * from to_del
    -> ;
+-----+-----------+
| id  | name      |
+-----+-----------+
| 001 | Lihua     |
| 002 | Liming    |
| 003 | Zhangchao |
| 004 | Zhaoli    |
| 005 | Lili      |
+-----+-----------+
5 rows in set (0.00 sec)

mysql> delete from to_del where id=2;
Query OK, 1 row affected (0.00 sec)

mysql> select * from to_del;
+-----+-----------+
| id  | name      |
+-----+-----------+
| 001 | Lihua     |
| 003 | Zhangchao |
| 004 | Zhaoli    |
| 005 | Lili      |
+-----+-----------+
4 rows in set (0.00 sec)

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删除整表数据

与UPDATE SET语句相同,DELETE FROM语句也可以删除整表的数据;

-- 查看表内数据 删除前
mysql> select * from to_del;
+-----+-----------+
| id  | name      |
+-----+-----------+
| 001 | Lihua     |
| 003 | Zhangchao |
| 004 | Zhaoli    |
| 005 | Lili      |
+-----+-----------+
4 rows in set (0.00 sec)

-- 删除整表数据 
mysql> delete from to_del;
Query OK, 4 rows affected (0.00 sec)

-- 查看表内数据 删除后
mysql> select * from to_del;
Empty set (0.00 sec)

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截断表 (TRUNCATE)

语法:

TRUNCATE [TABLE] table_name;

截断表的操作与删除整表的作用类似也为清除整表内容(不作示例);
但存在不同;
同样以该表进行测试;

+-----+-----------+
| id  | name      |
+-----+-----------+
| 001 | Lihua     |
| 002 | Liming    |
| 003 | Zhangchao |
| 004 | Zhaoli    |
| 005 | Lili      |
+-----+-----------+

• 使用SHOW CREATE TABLE table_name \G 对表的详细信息进行查看;

  mysql> show create table to_del\G
  *************************** 1. row ***************************
         Table: to_del
  Create Table: CREATE TABLE `to_del` (
  `id` int(3) unsigned zerofill NOT NULL AUTO_INCREMENT COMMENT 'id增加主键约束',
  `name` varchar(20) NOT NULL COMMENT 'name增加唯一键约束',
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
  ) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)
  

  对该表进行查看时自增AUTO_INCREMENT关键字为6;

• 使用DELETE FROM删除整表;

  mysql> delete from to_del;
  Query OK, 5 rows affected (0.01 sec)
  
  mysql> show create table to_del\G
  *************************** 1. row ***************************
       Table: to_del
  Create Table: CREATE TABLE `to_del` (
  `id` int(3) unsigned zerofill NOT NULL AUTO_INCREMENT COMMENT 'id增加主键约束',
  `name` varchar(20) NOT NULL COMMENT 'name增加唯一键约束',
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
  ) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)
  

  使用DELETE FROM对整表进行删除时其AUTO_INCREMENT关键字并没有改变;

• 使用TRUNCATE对整表进行截断操作;

  mysql> truncate to_del;
  Query OK, 0 rows affected (0.00 sec)
  
  mysql> show create table to_del\G
  *************************** 1. row ***************************
       Table: to_del
  Create Table: CREATE TABLE `to_del` (
  `id` int(3) unsigned zerofill NOT NULL AUTO_INCREMENT COMMENT 'id增加主键约束',
  `name` varchar(20) NOT NULL COMMENT 'name增加唯一键约束',
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
  ) ENGINE=MyISAM DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)
  

  而使用TRUNCATE对表进行截断操作时此时该表的AUTO_INCREMENT关键字不见了;

综上所述可以确定:
1. TRUNCATE只能对整表进行操作,且不能像DELETE对部分数据进行操作;
2. 若是表中存在AUTO_INCREMENT自增约束时,TRUNCATE的截断表操作将会重置AUTO_INCREMENT;
除此之外,对于DELETE来说,TRUNCATE的删除操作会较快,因为TRUNCATE在操作删除数据时并不经过真正的事务,所以无法回滚;

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总结

对于对表内数据的操作来说,无论是UPDATE还是DELETE与TRUNCATE对整表进行操作都应该注意;
尽量在对表内数据修改或删除前对需要修改\删除的数据使用 SELETC进行查看,避免误操作;