【MySQL】基本查询(下)

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本篇主题：【MySQL】基本查询(下)
发布时间：2025.2.18
隶属专栏：MySQL

在这里插入图片描述

Update

语法

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

对查询到的结果进行列值更新

案例

将孙悟空同学的数学成绩变更为 80 分

mysql> select name, math from exam_result;
+-----------+------+
| name      | math |
+-----------+------+
| 唐三藏    |   98 |
| 孙悟空    |   78 |
| 猪悟能    |   98 |
| 曹孟德    |   84 |
| 刘玄德    |   85 |
| 孙权      |   73 |
| 宋公明    |   65 |
+-----------+------+
7 rows in set (0.00 sec)mysql> update exam_result set math=80 where name='孙悟空';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0mysql> select name, math from exam_result;
+-----------+------+
| name      | math |
+-----------+------+
| 唐三藏    |   98 |
| 孙悟空    |   80 |
| 猪悟能    |   98 |
| 曹孟德    |   84 |
| 刘玄德    |   85 |
| 孙权      |   73 |
| 宋公明    |   65 |
+-----------+------+
7 rows in set (0.00 sec)

将曹孟德同学的数学成绩变更为 60 分，语文成绩变更为 70 分

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      82 |   84 |      67 |
|  5 | 刘玄德    |      55 |   85 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)mysql> update exam_result set math=60, chinese=70  where name='曹孟德';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      70 |   60 |      67 |
|  5 | 刘玄德    |      55 |   85 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

将总成绩倒数前三的 3 位同学的数学成绩加上 30 分

mysql> select name, math+chinese+english total from exam_result order by total asc limit 3;
+-----------+-------+
| name      | total |
+-----------+-------+
| 宋公明    |   170 |
| 刘玄德    |   185 |
| 曹孟德    |   197 |
+-----------+-------+
3 rows in set (0.00 sec)mysql> update exam_result set math=math+30 order by chinese+math+english limit 3;
Query OK, 3 rows affected (0.02 sec)
Rows matched: 3  Changed: 3  Warnings: 0mysql> select name, math+chinese+english total from exam_result;
+-----------+-------+
| name      | total |
+-----------+-------+
| 唐三藏    |   221 |
| 孙悟空    |   244 |
| 猪悟能    |   276 |
| 曹孟德    |   227 |
| 刘玄德    |   215 |
| 孙权      |   221 |
| 宋公明    |   200 |
+-----------+-------+
7 rows in set (0.00 sec)

将所有同学的语文成绩更新为原来的 2 倍

注意：更新全表的语句慎用！

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      70 |   90 |      67 |
|  5 | 刘玄德    |      55 |  115 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   95 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)mysql> update exam_result set chinese=chinese*2;
Query OK, 7 rows affected (0.00 sec)
Rows matched: 7  Changed: 7  Warnings: 0mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  2 | 孙悟空    |     174 |   80 |      77 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  5 | 刘玄德    |     110 |  115 |      45 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

Delete

删除数据

语法

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

案例

删除孙悟空同学的考试成绩

mysql> select * from exam_result where name='孙悟空';
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  2 | 孙悟空    |     174 |   80 |      77 |
+----+-----------+---------+------+---------+
1 row in set (0.00 sec)mysql> delete from exam_result where name='孙悟空';
Query OK, 1 row affected (0.00 sec)mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  5 | 刘玄德    |     110 |  115 |      45 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
6 rows in set (0.00 sec)

删除倒数第一名

mysql> select name, chinese+math+english total from exam_result order by total asc limit 1;
+-----------+-------+
| name      | total |
+-----------+-------+
| 刘玄德    |   270 |
+-----------+-------+
1 row in set (0.00 sec)mysql> delete from exam_result order by chinese+math+english asc limit 1;
Query OK, 1 row affected (0.01 sec)mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
5 rows in set (0.00 sec)

删除整张表数据

注意：删除整表操作要慎用！
创建表结构插入数据并查看。

mysql> CREATE TABLE for_delete (-> id INT PRIMARY KEY AUTO_INCREMENT,-> name VARCHAR(20)-> );
Query OK, 0 rows affected (0.02 sec)mysql> INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)mysql> show create table for_delete\G
*************************** 1. row ***************************Table: for_delete
Create Table: CREATE TABLE `for_delete` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

删除数据

mysql> delete from for_delete;
Query OK, 3 rows affected (0.01 sec)mysql> select * from for_delete;
Empty set (0.00 sec)

插入数据并查看表结构

mysql> insert into for_delete (name) values ('D');
Query OK, 1 row affected (0.00 sec)mysql> show create table for_delete\G
*************************** 1. row ***************************Table: for_delete
Create Table: CREATE TABLE `for_delete` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  4 | D    |
+----+------+
1 row in set (0.00 sec)

截断表

语法

TRUNCATE [TABLE] table_name

注意：这个操作慎用

只能对整表操作，不能像 DELETE 一样针对部分数据操作；
实际上 MySQL 不对数据操作，所以比 DELETE 更快，但是TRUNCATE在删除数据的时候，并不经过真正的事物，所以无法回滚
会重置 AUTO_INCREMENT 项

案例

创建表结构并插入数据

mysql> CREATE TABLE for_truncate (-> id INT PRIMARY KEY AUTO_INCREMENT,-> name VARCHAR(20)-> );
Query OK, 0 rows affected (0.01 sec)mysql> INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

截断整表数据，注意影响行数是 0，所以实际上没有对数据真正操作

mysql> truncate for_truncate;
Query OK, 0 rows affected (0.03 sec)mysql> select * from for_truncate;
Empty set (0.00 sec)

再插入一条数据，自增 id 在重新增长

mysql> insert into for_truncate (name) values ('D');
Query OK, 1 row affected (0.01 sec)mysql> show create table for_truncate\G
*************************** 1. row ***************************Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | D    |
+----+------+
1 row in set (0.00 sec)

插入查询结果

语法

INSERT INTO table_name [(column [, column ...])] SELECT ...

案例

删除表中的的重复复记录，重复的数据只能有一份

创建原数据表并插入数据

mysql> CREATE TABLE duplicate_table (id int, name varchar(20));
Query OK, 0 rows affected (0.01 sec)mysql> INSERT INTO duplicate_table VALUES-> (100, 'aaa'),-> (100, 'aaa'),-> (200, 'bbb'),-> (200, 'bbb'),-> (200, 'bbb'),-> (300, 'ccc');
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0mysql> desc duplicate_table;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

创建一张空表 no_duplicate_table，结构和 duplicate_table 一样

mysql> create table no_duplicate_table like duplicate_table;
Query OK, 0 rows affected (0.02 sec)mysql> desc no_duplicate_table;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

将 duplicate_table 的去重数据插入到 no_duplicate_table

mysql> insert into no_duplicate_table select distinct * from duplicate_table;
Query OK, 3 rows affected (0.03 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> select * from no_duplicate_table;
+------+------+
| id   | name |
+------+------+
|  100 | aaa  |
|  200 | bbb  |
|  300 | ccc  |
+------+------+
3 rows in set (0.00 sec)

通过重命名表，实现原子的去重操作

mysql> rename table duplicate_table to old_duplicate_table;
Query OK, 0 rows affected (0.02 sec)mysql> rename table no_duplicate_table to duplicate_table;
Query OK, 0 rows affected (0.01 sec)

查看最终结果

mysql> select * from duplicate_table;
+------+------+
| id   | name |
+------+------+
|  100 | aaa  |
|  200 | bbb  |
|  300 | ccc  |
+------+------+
3 rows in set (0.01 sec)

为什么最后是通过rename的方式进行的？
就是单纯的等一切都就绪了，然后统一放入、更新、生效等。确保操作是原子的。

聚合函数

函数介绍

函数	说明
COUNT([DISTINCT] expr)	返回查询到的数据的数量
SUM([DISTINCT] expr)	返回查询到的数据的总和，不是数字没有意义
AVG([DISTINCT] expr)	返回查询到的数据的平均值，不是数字没有意义
MAX([DISTINCT] expr)	返回查询到的数据的最大值，不是数字没有意义
MIN([DISTINCT] expr)	返回查询到的数据的最小值，不是数字没有意义

案例

统计班级共有多少同学

使用 * 做统计，不受 NULL 影响

mysql> select count(*) from exam_result;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.03 sec)

使用表达式做统计

mysql> select count(1) from exam_result;
+----------+
| count(1) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

统计班级收集的 qq 号有多少

NULL 不会计入结果

mysql> select count(qq) from students;
+-----------+
| count(qq) |
+-----------+
|         9 |
+-----------+
1 row in set (0.00 sec)

统计本次考试的数学成绩分数个数

COUNT(math) 统计的是全部成绩

mysql> select count(math) from exam_result;
+-------------+
| count(math) |
+-------------+
|           5 |
+-------------+
1 row in set (0.00 sec)

COUNT(DISTINCT math) 统计的是去重成绩数量

mysql> select count(distinct math) from exam_result;
+----------------------+
| count(distinct math) |
+----------------------+
|                    4 |
+----------------------+
1 row in set (0.02 sec)

统计数学成绩总分

如果没有结果，则返回 NULL

mysql> select sum(math) from exam_result;
+-----------+
| sum(math) |
+-----------+
|       454 |
+-----------+
1 row in set (0.00 sec)mysql> select sum(math) from exam_result where math<60;
+-----------+
| sum(math) |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

统计平均总分

mysql> select avg(chinese+math+english) 平均总分 from exam_result;
+--------------+
| 平均总分     |
+--------------+
|          303 |
+--------------+
1 row in set (0.00 sec)

返回英语最高分

mysql> select max(english) from exam_result;
+--------------+
| max(english) |
+--------------+
|           90 |
+--------------+
1 row in set (0.01 sec)

返回 > 70 分以上的英语最低分

mysql> select min(english) from exam_result where english>70;
+--------------+
| min(english) |
+--------------+
|           78 |
+--------------+
1 row in set (0.00 sec)

group by子句的使用

在select中使用group by 子句可以对指定列进行分组查询

语法

select column1, column2, .. from table group by column;

分组的目的是为了进行分组以后，方便进行聚合统计

having和where

where：对具体的任意列进行条件筛选
having：对分组聚合之后的结果进行条件筛选

案例

准备工作，创建一个雇员信息表（来自oracle 9i的经典测试表）

EMP员工表
DEPT部门表
SALGRADE工资等级表

具体代码可私我

如何显示每个部门的平均工资和最高工资

mysql> select deptno,avg(sal),max(sal) from emp group by deptno;
+--------+-------------+----------+
| deptno | avg(sal)    | max(sal) |
+--------+-------------+----------+
|     10 | 2916.666667 |  5000.00 |
|     20 | 2175.000000 |  3000.00 |
|     30 | 1566.666667 |  2850.00 |
+--------+-------------+----------+
3 rows in set (0.00 sec)

显示每个部门的每种岗位的平均工资和最低工资

mysql> select avg(sal),min(sal),job, deptno from emp group by deptno, job;
+-------------+----------+-----------+--------+
| avg(sal)    | min(sal) | job       | deptno |
+-------------+----------+-----------+--------+
| 1300.000000 |  1300.00 | CLERK     |     10 |
| 2450.000000 |  2450.00 | MANAGER   |     10 |
| 5000.000000 |  5000.00 | PRESIDENT |     10 |
| 3000.000000 |  3000.00 | ANALYST   |     20 |
|  950.000000 |   800.00 | CLERK     |     20 |
| 2975.000000 |  2975.00 | MANAGER   |     20 |
|  950.000000 |   950.00 | CLERK     |     30 |
| 2850.000000 |  2850.00 | MANAGER   |     30 |
| 1400.000000 |  1250.00 | SALESMAN  |     30 |
+-------------+----------+-----------+--------+
9 rows in set (0.00 sec)

显示平均工资低于2000的部门和它的平均工资

统计各个部门的平均工资

mysql> select avg(sal) from emp group by deptno;
+-------------+
| avg(sal)    |
+-------------+
| 2916.666667 |
| 2175.000000 |
| 1566.666667 |
+-------------+
3 rows in set (0.00 sec)

having和group by配合使用，对group by结果进行过滤

mysql> select avg(sal) as myavg from emp group by deptno having myavg<2000;
+-------------+
| myavg       |
+-------------+
| 1566.666667 |
+-------------+
1 row in set (0.00 sec)

结语

不要单纯的认为，只有磁盘上的表结构导入到MySQL，真实存在的表才叫表。中间筛选出来的，包括最终结果，都可以看成逻辑上的表。
MySQL一切皆表
未来我们只要能够处理好单表的CURD，所有sql场景，我们全部都能用统一的方式进行。

⚠️ 写在最后：以上内容是我在学习以后得一些总结和概括，如有错误或者需要补充的地方欢迎各位大佬评论或者私信我交流！！！

【MySQL】基本查询(下)

目录

Update

语法

案例

Delete

删除数据

语法

案例

截断表

语法

案例

插入查询结果

语法

案例

聚合函数

函数介绍

案例

group by子句的使用

语法

having和where

案例

结语

相关资讯

热文排行

最新新闻

推荐新闻

热搜词

【MySQL】 基本查询(下)

目录

Update

语法

案例

Delete

删除数据

语法

案例

截断表

语法

案例

插入查询结果

语法

案例

聚合函数

函数介绍

案例

group by子句的使用

语法

having和where

案例

结语

相关资讯

热文排行

最新新闻

推荐新闻

热搜词

【MySQL】基本查询(下)