8.Mysql 常用函数和基础查询
目录
-
- Mysql 常用函数
-
- 数学函数
- 字符串函数
- 日期函数
- json 函数
- 聚合函数
- 简单查询
-
- 直接查询
- 去重查询
- 条件查询
- 排序查询
- 分页查询
- 分组查询
Mysql 函数参考和扩展: Mysql 官网
Mysql 语法执行顺序如下，一定要清楚！！！运算符相关，可前往 Mysql 基础语法和执行顺序扩展。
(8) select (9) distinct (11)<columns_name list>
(1) from <left_table>
(3) <join_type> join <right_table>
(2) on <join_condition>
(4) where <where_condition>
(5) group by <group_by columns_name list>
(6) with <rollup>
(7) having <having_condition>
(10) order by <order_by columns_name list>
(12) limit <[offset，] rows>
;
Mysql 常用函数
数学函数
字符串函数
日期函数
json 函数
聚合函数
简单查询
数据准备
create database sql_test1;
use sql_test1;
create table if not exists `user_info`
(
`id` int not null,
`device_id` int not null comment '用户id',
`gender` varchar(14) not null comment '性别',
`age` int comment '年龄',
`university` varchar(32) not null comment '大学',
`gpa` float comment '得分',
`active_days_within_30` int comment '30天内活跃天数'
);
truncate table user_info;
insert into user_info values(1,2138,'male',21,'北京大学',3.4,7);
insert into user_info values(2,3214,'male',null,'复旦大学',4.0,15);
insert into user_info values(3,6543,'female',20,'北京大学',3.2,12);
insert into user_info values(4,2315,'female',23,'浙江大学',3.6,5);
insert into user_info values(5,5432,'male',25,'山东大学',3.8,20);
insert into user_info values(6,2131,'male',28,'山东大学',3.3,15);
insert into user_info values(7,4321,'male',28,'复旦大学',3.6,9);
直接查询
查询所有列、指定字段和字段起别名
select * from user_info; -- 查询所有列
+----+-----------+--------+------+--------------+------+-----------------------+
| id | device_id | gender | age | university | gpa | active_days_within_30 |
+----+-----------+--------+------+--------------+------+-----------------------+
| 1 | 2138 | male | 21 | 北京大学 | 3.4 | 7 |
| 2 | 3214 | male | NULL | 复旦大学 | 4 | 15 |
| 3 | 6543 | female | 20 | 北京大学 | 3.2 | 12 |
| 4 | 2315 | female | 23 | 浙江大学 | 3.6 | 5 |
| 5 | 5432 | male | 25 | 山东大学 | 3.8 | 20 |
| 6 | 2131 | male | 28 | 山东大学 | 3.3 | 15 |
| 7 | 4321 | male | 28 | 复旦大学 | 3.6 | 9 |
+----+-----------+--------+------+--------------+------+-----------------------+
select device_id,gender,age,university
from user_info;-- 查询指定字段
+-----------+--------+------+--------------+
| device_id | gender | age | university |
+-----------+--------+------+--------------+
| 2138 | male | 21 | 北京大学 |
| 3214 | male | NULL | 复旦大学 |
| 6543 | female | 20 | 北京大学 |
| 2315 | female | 23 | 浙江大学 |
| 5432 | male | 25 | 山东大学 |
| 2131 | male | 28 | 山东大学 |
| 4321 | male | 28 | 复旦大学 |
+-----------+--------+------+--------------+
select device_id,gpa,
round(gpa*rand(),1) as gpa_adj
from user_info; -- 生成新计算列并起别名
+-----------+------+---------+
| device_id | gpa | gpa_adj |
+-----------+------+---------+
| 2138 | 3.4 | 2.3 |
| 3214 | 4 | 2.3 |
| 6543 | 3.2 | 2.8 |
| 2315 | 3.6 | 2.4 |
| 5432 | 3.8 | 2.4 |
| 2131 | 3.3 | 0.9 |
| 4321 | 3.6 | 1.3 |
+-----------+------+---------+
去重查询
select distinct university
from user_info;-- 查看去重后有哪些大学
+--------------+
| university |
+--------------+
| 北京大学 |
| 复旦大学 |
| 浙江大学 |
| 山东大学 |
+--------------+
条件查询
select *
from user_info
where age is null;-- 查询年龄不为null的数据
+----+-----------+--------+------+--------------+------+-----------------------+
| id | device_id | gender | age | university | gpa | active_days_within_30 |
+----+-----------+--------+------+--------------+------+-----------------------+
| 2 | 3214 | male | NULL | 复旦大学 | 4 | 15 |
+----+-----------+--------+------+--------------+------+-----------------------+
select device_id
from user_info
where university = '北京大学'
and gender = 'male';-- 查询北京大学男性有哪些用户
+-----------+
| device_id |
+-----------+
| 2138 |
+-----------+
排序查询
select *
from user_info
order by gender,age desc;-- 查询性别升序，年龄降序的记录
+----+-----------+--------+------+--------------+------+-----------------------+
| id | device_id | gender | age | university | gpa | active_days_within_30 |
+----+-----------+--------+------+--------------+------+-----------------------+
| 4 | 2315 | female | 23 | 浙江大学 | 3.6 | 5 |
| 3 | 6543 | female | 20 | 北京大学 | 3.2 | 12 |
| 6 | 2131 | male | 28 | 山东大学 | 3.3 | 15 |
| 7 | 4321 | male | 28 | 复旦大学 | 3.6 | 9 |
| 5 | 5432 | male | 25 | 山东大学 | 3.8 | 20 |
| 1 | 2138 | male | 21 | 北京大学 | 3.4 | 7 |
| 2 | 3214 | male | NULL | 复旦大学 | 4 | 15 |
+----+-----------+--------+------+--------------+------+-----------------------+
分页查询
select *
from user_info
limit 1;-- 查询出一条数据
+----+-----------+--------+------+--------------+------+-----------------------+
| id | device_id | gender | age | university | gpa | active_days_within_30 |
+----+-----------+--------+------+--------------+------+-----------------------+
| 1 | 2138 | male | 21 | 北京大学 | 3.4 | 7 |
+----+-----------+--------+------+--------------+------+-----------------------+
select *
from user_info
order by gpa desc
limit 1;-- 查询得分最高的一个用户
+----+-----------+--------+------+--------------+------+-----------------------+
| id | device_id | gender | age | university | gpa | active_days_within_30 |
+----+-----------+--------+------+--------------+------+-----------------------+
| 2 | 3214 | male | NULL | 复旦大学 | 4 | 15 |
+----+-----------+--------+------+--------------+------+-----------------------+
分组查询
select university,
count(distinct device_id) as user_cnt,
round(avg(gpa),2) as avg_gpa,
max(gpa) as max_gpa,
min(gpa) as min_gpa,
group_concat(distinct age order by age separator '、') as age_distr
from user_info
group by university;-- 查询每个大学的用户数、平均得分、最大的分、最小得分和年龄分布情况
+--------------+----------+---------+---------+---------+-----------+
| university | user_cnt | avg_gpa | max_gpa | min_gpa | age_distr |
+--------------+----------+---------+---------+---------+-----------+
| 北京大学 | 2 | 3.3 | 3.4 | 3.2 | 20、21 |
| 复旦大学 | 2 | 3.8 | 4 | 3.6 | 28 |
| 山东大学 | 2 | 3.55 | 3.8 | 3.3 | 25、28 |
| 浙江大学 | 1 | 3.6 | 3.6 | 3.6 | 23 |
+--------------+----------+---------+---------+---------+-----------+