Python学习之复习MySQL-Day3（DQL）

---

文章声明⭐⭐⭐

1. 该文章为我（有编程语言基础，非编程小白）的 MySQL复习笔记
2. 知识来源为 B站UP主（黑马程序员）的MySQL课程视频，归纳为自己的语言与理解记录于此并加以实践
3. 此前我已经学习过了MySQL，现在是在复习阶段，所以不是面向小白的教学文章
4. 不出意外的话，我大抵会 持续更新
5. 想要了解前端开发（技术栈大致有：Vue2/3、微信小程序、uniapp、HarmonyOS、NodeJS、Typescript）与Python的小伙伴，可以关注我！谢谢大家！

---

让我们开始今天的学习吧！

DQL简介

DQL英文全称Data Query Language，即数据查询语言，用来查询表中数据

完整语法如下：

select 字段列表 from 表名列表 where 条件列表 group by 分组字段列表 having 分组后条件列表 order by 排序字段列表 limit 分页参数 

---

基本查询

查询多个/全部字段

mysql> select id,username from emp;
+------+----------+
| id   | username |
+------+----------+
|    1 | Richie   |
|    2 | Taylor   |
|    3 | Mike     |
+------+----------+
3 rows in set (0.00 sec)mysql> select * from emp; # 数据量大的时候尽量不要使用，因为效率比较低
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    3 | Mike     |   45 | 男     |
+------+----------+------+--------+
3 rows in set (0.00 sec)

设置别名

设置别名的好处在于，我们可以自定义字段名，可以更直观的观察数据

mysql> select username as name,gender as sex from emp;
+--------+------+
| name   | sex  |
+--------+------+
| Richie | 男   |
| Taylor | 女   |
| Mike   | 男   |
+--------+------+
3 rows in set (0.00 sec)mysql> select username name,gender sex from emp; # as可以省略
+--------+------+
| name   | sex  |
+--------+------+
| Richie | 男   |
| Taylor | 女   |
| Mike   | 男   |
+--------+------+
3 rows in set (0.00 sec)

去除重复记录

mysql> select distinct gender '性别' from emp; # 使用关键字distinc来去除重复记录
+------+
| 性别 |
+------+
| 男   |
| 女   |
+------+
2 rows in set (0.00 sec)

---

条件查询

条件查询介绍

比较运算符	功能
>	大于
>=	大于等于
<	小于
<=	小于等于
=	等于
<> 或 !=	不等于
between … and …	在某个范围之内（含最大最小值）
in(…)	在in之后的列表中的值，就是多选一的意思
like 占位符	模糊匹配（_匹配单个字符，%匹配任意个字符）
is null	是null

逻辑运算符	功能
and 或 &&	并且（多个条件同时成立）
or 或 ||	或者（多个条件任意一个成立）
not 或 !	非（不是）

实例演示

mysql> select * from emp where age = 88; # 查询年龄等于88的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    5 | 张三     |   88 |        |
+------+----------+------+--------+
1 row in set (0.00 sec)mysql> select * from emp where age < 30; # 查询年龄小于30的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    4 |          |   25 | 女     |
+------+----------+------+--------+
2 rows in set (0.00 sec)mysql> select * from emp where age <= 25; # 查询年龄小于等于25的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    4 |          |   25 | 女     |
+------+----------+------+--------+
2 rows in set (0.00 sec)mysql> select * from emp where username = '' or username is null; # 查询没有姓名的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    4 |          |   25 | 女     |
+------+----------+------+--------+
1 row in set (0.00 sec)mysql> select * from emp where not (username = '' or username is null); # 查询有姓名的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    3 | Mike     |   45 | 男     |
|    5 | 张三     |   88 |        |
+------+----------+------+--------+
4 rows in set (0.00 sec)mysql> select * from emp where age != 45; # 查询年龄不等于45的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    4 |          |   25 | 女     |
|    5 | 张三     |   88 |        |
+------+----------+------+--------+
4 rows in set (0.00 sec)mysql> select * from emp where age between 20 and 40; # 查询年龄在20-40之间的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    4 |          |   25 | 女     |
+------+----------+------+--------+
3 rows in set (0.00 sec)mysql> select * from emp where gender = '女' and age < 30; # 查询性别为女且年龄小于30的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    4 |          |   25 | 女     |
+------+----------+------+--------+
1 row in set (0.00 sec)mysql> select * from emp where gender = '女' and age < 30; # 查询年龄为21或者25或者34的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    4 |          |   25 | 女     |
+------+----------+------+--------+
1 row in set (0.00 sec)mysql> select * from emp where age in (21,25,34); # 查询年龄为21或者25或者34的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    4 |          |   25 | 女     |
+------+----------+------+--------+
3 rows in set (0.00 sec)mysql> select * from emp where username like '__'; # 查询姓名为两个字的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    5 | 张三     |   88 |        |
+------+----------+------+--------+
1 row in set (0.00 sec)mysql> select * from emp where username like '%e'; # 查询姓名最后是e的员工
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    3 | Mike     |   45 | 男     |
+------+----------+------+--------+
2 rows in set (0.00 sec)

---

聚合函数

什么是聚合函数？

聚合函数用于将一列数据作为一个整体，进行纵向的计算

注意：null不参与聚合函数的运算

常见的聚合函数

函数	功能
count	统计数量
max	最大值
min	最小值
avg	平均值
sum	求和

实例演示

mysql> select * from emp;
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   34 | 女     |
|    3 | Mike     |   45 | 男     |
|    4 |          |   25 | 女     |
|    5 | 张三     |   88 |        |
+------+----------+------+--------+
5 rows in set (0.00 sec)mysql> select count(id) from emp; # 统计有多少条包含id字段的记录，不统计null
+-----------+
| count(id) |
+-----------+
|         5 |
+-----------+
1 row in set (0.00 sec)mysql> select avg(age) from emp; # 统计员工的平均年龄
+----------+
| avg(age) |
+----------+
|  42.6000 |
+----------+
1 row in set (0.03 sec)mysql> select max(age) from emp; # 统计员工的最大年龄
+----------+
| max(age) |
+----------+
|       88 |
+----------+
1 row in set (0.00 sec)mysql> select min(age) from emp; # 统计员工的最小年龄
+----------+
| min(age) |
+----------+
|       21 |
+----------+
1 row in set (0.00 sec)mysql> select sum(age) from emp where gender = '男'; # 统计男员工的年龄之和
+----------+
| sum(age) |
+----------+
|       66 |
+----------+
1 row in set (0.00 sec)

---

分组查询

分组查询语法

select 字段列表 from 表名 [where 条件] group by 分组字段名 [having 分组后过滤的条件]

where 和 having 的区别

1. 执行时机不同：where 是在 group by 分组之前进行过滤，不满足 where 条件的，不参与 group by 分组；而 having 则是在 group by 分组之后过滤的
2. 判断条件不同：where 不能对聚合函数进行判断过滤，而 having 则可以

注意：执行顺序为 where > 聚合函数 > having

实例演示

mysql> select * from emp;
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   29 | 女     |
|    3 | Mike     |   38 | 男     |
|    4 | Jack     |   88 | 男     |
|    5 | Joker    |   56 | 男     |
|    6 | Nancy    |   18 | 女     |
+------+----------+------+--------+
6 rows in set (0.00 sec)mysql> select gender,count(*) from emp group by gender; # 根据性别分组，统计男员工和女员工的数量
+--------+----------+
| gender | count(*) |
+--------+----------+
| 男     |        4 |
| 女     |        2 |
+--------+----------+
2 rows in set (0.00 sec)mysql> select gender,avg(age) from emp group by gender; # 根据性别分组，统计男员工和女员工的平均年龄
+--------+----------+
| gender | avg(age) |
+--------+----------+
| 男     |  50.7500 |
| 女     |  23.5000 |
+--------+----------+
2 rows in set (0.03 sec)mysql> select gender,count(*) from emp where age < 60 group by gender having count(*) >= 3; # 查询年龄小于60的员工，并根据性别分组，获取员工数量大于等于3的性别
+--------+----------+
| gender | count(*) |
+--------+----------+
| 男     |        3 |
+--------+----------+
1 row in set (0.00 sec)mysql> select gender,count(*) num from emp where age < 60 group by gender having num >= 3; # 还可以在上一个例题中起别名
+--------+-----+
| gender | num |
+--------+-----+
| 男     |   3 |
+--------+-----+
1 row in set (0.03 sec)

---

排序查询

语法

select 字段列表 from 表名 order by 字段1 排序方式1,字段2 排序方式2 ... ;

排序方式

• 升序：asc（ascend，此为默认值）
• 降序：desc（descend）

实例演示

mysql> select * from emp;
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   29 | 女     |
|    3 | Mike     |   38 | 男     |
|    4 | Jack     |   88 | 男     |
|    5 | Joker    |   56 | 男     |
|    6 | Nancy    |   18 | 女     |
+------+----------+------+--------+
6 rows in set (0.00 sec)mysql> select username '姓名',age '年龄',gender '性别' from emp where gender = '男' order by age; # 根据 年龄对男员工进行升序排序
+--------+------+------+
| 姓名   | 年龄 | 性别 |
+--------+------+------+
| Richie |   21 | 男   |
| Mike   |   38 | 男   |
| Joker  |   56 | 男   |
| Jack   |   88 | 男   |
+--------+------+------+
4 rows in set (0.00 sec)

---

分页查询

语法

select 字段列表 from 表名 limit 起始索引 , 查询记录数;

注意：

• 起始索引从 0 开始计算，起始索引 = （要查询的页码数 - 1）* 每页显示记录数
• 如果查询的是第一页，起始索引可以省略

实例演示

mysql> select * from emp;
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   29 | 女     |
|    3 | Mike     |   38 | 男     |
|    4 | Jack     |   88 | 男     |
|    5 | Joker    |   56 | 男     |
|    6 | Nancy    |   18 | 女     |
|    7 | Linken   |   57 | 男     |
+------+----------+------+--------+
7 rows in set (0.00 sec)mysql> select * from emp limit 0 , 4; # 查询第一页员工数据，每页展示4条数据
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   29 | 女     |
|    3 | Mike     |   38 | 男     |
|    4 | Jack     |   88 | 男     |
+------+----------+------+--------+
4 rows in set (0.00 sec)mysql> select * from emp limit 4; # 查询第一页数据时可以简写
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    1 | Richie   |   21 | 男     |
|    2 | Taylor   |   29 | 女     |
|    3 | Mike     |   38 | 男     |
|    4 | Jack     |   88 | 男     |
+------+----------+------+--------+
4 rows in set (0.00 sec)mysql> select * from emp limit 5,5; # 查询第二页数据，每页展示5条数据
+------+----------+------+--------+
| id   | username | age  | gender |
+------+----------+------+--------+
|    6 | Nancy    |   18 | 女     |
|    7 | Linken   |   57 | 男     |
+------+----------+------+--------+
2 rows in set (0.00 sec)

---

综合练习

# 查询年龄为20，21，22，23岁的女性员工信息
select * from emp where age in (20,21,22,23) && gender = '女';
# 查询性别为男，并且年龄在 20-40 （不包含20岁包含40岁）岁且姓名长度等于4的员工信息
select * from emp where gender = '男' && age between 21 and 40 && name like '____';
# 统计员工表中年龄小于60岁的男性员工和女性员工的人数
select gender , count(*) from emp where age < 60 group by gender
# 查询所有年龄小于等于35岁的员工姓名和年龄，并对查询结果按年龄升序排序，如果年龄相同按入职时间降序排序
select name,age from emp where age <= 35 order by age asc,entrytime desc;
# 查询性别为男，并且年龄在 20-40岁（包含20、40岁）以内的前五名员工信息
select * from emp where gender = '男' && age between 20 and 40 limit 5;