原文链接：https://www.cnblogs.com/168vincent/p/9201089.html

--数据库操作前的准备
-- 创建数据库
-- create database python_test_1 charset=utf8;

-- 使用数据库
-- use python_test_1;

-- students表
-- create table students(
-- id int unsigned primary key auto_increment not null,
-- name varchar(20) default ‘‘,
-- age tinyint unsigned default 0,
-- height decimal(5,2),
-- gender enum(‘男‘,‘女‘,‘中性‘,‘保密‘) default ‘保密‘,
-- cls_id int unsigned default 0,
-- is_delete bit default 0
-- );

-- classes表
-- create table classes (
-- id int unsigned auto_increment primary key not null,
-- name varchar(30) not null
-- );

-- 查询练习
-- 查询所有字段
-- select * from 表名;
select * from students;

-- 查询指定字段
-- select 列1,列2,... from 表名;
select name,age from students;

-- 使用 as 给字段起别名
-- select 字段 as 名字.... from 表名;
select name as ‘姓名‘,age from students;

 

-- select 表名.字段 .... from 表名;
select students.name from students;

-- 可以通过 as 给表起别名
-- select 别名.字段 .... from 表名 as 别名;
select * from students as s;

select s.name from students as s;

-- 消除重复行(查性别)

-- distinct 字段 不要记有个印象
select distinct gender from students;

> < >= <= != <>
-- 条件查询
-- 比较运算符
-- select .... from 表名 where .....
-- >
-- 查询年纪大于18岁的信息
select * from students where age > 18;

-- <
-- 查询年纪小于18岁的信息
select * from students where age < 18;

-- >=
-- <=
-- 查询小于等于18岁的信息
select * from students where age <= 18;

-- =
-- 查询年龄为18岁的所有学生的名字
select * from students where age = 18;

 

-- != 或者 <>
-- 查询年龄不为18岁的所有学生的名字
select * from students where age != 18;
-- select * from students where age <> 18;
and or not
-- 逻辑运算符
-- and
-- 18和28之间的所以学生信息
select * from students where age > 18 and age < 28;

-- 18岁以上的女性
select * from students where age > 18 and gender = ‘女‘;

-- or
-- 18以上或者身高高过180(包含)以上
select * from students where age > 18 or height >= 180;

-- not
-- 不在 18岁以上的女性 这个范围内的信息
-- select * from students where not (age>18 and gender=2);
select * from students where not age > 18 and gender= "女";

select * from students where not (age > 18 and gender = "女");

like % _
-- 模糊查询(where name like 要查询的数据)
-- like
-- % 替换任意个
-- _ 替换1个
-- 查询姓名中 以 "小" 开始的名字
select * from students where name like ‘小%‘;

-- 查询姓名中 有 "小" 所有的名字
select * from students where name like ‘%小%‘;

-- 查询有2个字的名字
select * from students where name like ‘__‘;

-- 查询有3个字的名字
select * from students where name like ‘___‘;

 

-- 查询至少有2个字的名字
select * from students where name like ‘__%‘;

select * from students where name not like "__";

-- 范围查询
-- in (1, 3, 8)表示在一个非连续的范围内
-- 查询 年龄为18或34的姓名
select * from students where age = 18 or age = 34 ;
select * from students where age in (18,34);

-- not in 不非连续的范围之内
-- 年龄不是 18或34岁的信息
select * from students where age not in(18,34);

-- between ... and ...表示在一个连续的范围内
-- 查询 年龄在18到34之间的的信息
select * from students where age > 18 and age < 34;
-- between xxx and xxx
select * from students where age between 18 and 34; --between...and...这是包含两端的数据

-- not between ... and ...表示不在一个连续的范围内
-- 查询 年龄不在18到34之间的的信息

select * from students where age not between 18 and 34;

-- 空判断
-- 判空is null
-- 查询身高为空的信息
select * from students where height is null;

 

-- 判非空is not null
select * from students where height is not null;

order by 字段 asc,desc

-- 排序
-- order by 字段
-- asc
-- asc从小到大排列，即升序
-- desc
-- desc从大到小排序，即降序
-- 查询年龄在18到34岁之间的男性，按照年龄从小到大到排序
select * from students where (age between 18 and 34) and gender=‘男‘ order by age asc;

 

-- 查询年龄在18到34岁之间的女性，身高从高到矮排序
select * from students where (age between 18 and 34) and gender =‘女‘ order by height desc;

-- order by 多个字段
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
select * from students where (age between 18 and 34) and gender = ‘女‘ order by height desc,age asc;

-- 如果年龄也相同那么按照id从大到小排序
select * from students where (age between 18 and 34) and gender =‘女‘ order by height desc,age asc,id desc;
-- 排序有优先级,第一个主排序,后面是次排序,在保证主排序不变的情况下,能排就排,不排就算了

-- 聚合函数
-- 总数
-- count
-- 查询男性有多少人 count(字段) 要注意如果值有null那么不会进行计算
select count(*) from students where gender=‘男‘;

-- 最大值
-- max
-- 查询最大的年龄
select max(age) from students;

-- 查询女性的最高 身高
select max(height) from students where gender =‘女‘;

-- 最小值
-- min
select min(age) from students ;

-- 求和
-- sum
-- 计算所有人的年龄总和
select sum(age) from students;


-- 平均值
-- avg
-- 计算平均年龄
select avg(age) from students;

-- 计算平均年龄 sum(age)/count(*)
select sum(age)/count(*) from students;

-- 四舍五入 round(123.23 , 1) 保留1位小数
-- 计算所有人的平均年龄，保留2位小数
select round (avg(age),2) from students;

-- 计算男性的平均身高 保留2位小数
select round(avg(height),2) from students where gender=‘男‘;
select avg(height) from students where gender = ‘男‘;

-- 分组

-- group by
-- 按照性别分组,查询所有的性别
-- select 分组字段 from 表名 group by 分组字段;
select gender from students group by gender;

select 分组字段 from 表名 group by 分组字段;

-- 计算每种性别中的人数
select gender,count(*) from students group by gender;

 

-- group_concat(...)
-- 查询同种性别中的姓名
select gender,group_concat(name) from students group by gender;

-- 查询每组性别的平均年龄
select gender,avg(age) from students group by gender;

-- select * from students where
-- group by xxx having having用在分组条件

-- having(注意having和group by 连用 having后通常也要跟 聚合函数)
-- 查询平均年龄超过30岁的性别，以及姓名
select gender ,avg(age) from students group by gender having avg(age) > 30;

-- 查询每种性别中的人数多于2个的信息
select gender,count(*) from students group by gender having count(*) > 2;

 

-- with rollup 汇总的作用(了解)
--select gender,count(*) from students group by gender with rollup;
select gender,count(*) from students group by gender with rollup having count(*) >2;

--按性别分组,平均身高大160的女性组的名字
select gender,avg(height),group_concat(name) from students group by gender having avg(height) > 160 and gender=‘女‘;

-- limit 起始位置,个数, 这个一定要放在最后
-- 分页
-- limit start, count
-- limit 放在最后面(注意)

起始位置 = (页数-1)*每页的个数

-- 限制查询出来的数据个数
-- 查询前5个数据
select * from students limit 0,5;

-- 每页显示2个，第1个页面
select * from students limit 0,2;

-- 每页显示2个，第2个页面
select * from students limit 2,2;

-- 每页显示2个，第3个页面
select * from students limit 4,2;

-- 每页显示2个，第4个页面
select * from students limit 6,2;

 

-- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
select * from students order by age asc limit 6,2;
-- 如果重新排序了,那么会显示第一页

-- 连接查询
-- inner join ... on
-- select ... from 表A inner join 表B;
-- 查询 有能够对应班级的学生以及班级信息
select * from students inner join classes on students.cls_id = classes.id;

-- 按照要求显示姓名、班级
select students.name,classes.name from students inner join classes on students.cls_id = classes.id;

-- 给数据表起名字
select s.name,c.name from students as s inner join classes as c on s.cls_id = c.id;

-- 查询 有能够对应班级的学生以及班级信息，显示学生的所有信息 students.*，只显示班级名称 classes.name.
select students.* ,classes.name from students inner join classes on students.cls_id = classes.id;

-- 在以上的查询中，将班级名显示在第1列
select classes.name,students.* from students inner join classes on students.cls_id = classes.id;

-- 查询 有能够对应班级的学生以及班级信息, 按照班级名进行排序
select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc;

-- 当时同一个班级的时候，按照学生的id进行从小到大排序
select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc,students.id asc;

#如果是group by 条件使用having
#如果是inner join条件使用on
#其他都用where

-- left join
-- 查询每位学生对应的班级信息
select * from students left join classes on students.cls_id = classes.id;
左边的表不管在右边的表中是否找到数据,都显示

-- 查询没有对应班级信息的学生
select * from students left join classes on students.cls_id = classes.id where classes.name is null;

-- right join on
-- 将数据表名字互换位置，用left join完成

select * from students right join classes on students.cls_id = classes.id;

select * from classes right join students on students.cls_id = classes.id;

 

-- 子查询
-- 标量子查询: 子查询返回的结果是一个数据(一行一列)
-- 列子查询: 返回的结果是一列(一列多行)
-- 行子查询: 返回的结果是一行(一行多列)

-- 查询出高于平均身高的信息(height)
select avg(height) from students;

select * from students where height > 172;

select * from students where height > (select avg(height) from students);

 

-- 查询学生的班级号能够对应的 学生名字
select * from students where cls_id in (1,2);

select id from classes;

select * from students where cls_id in (select id from classes);
省市区三级联动

--数据操作前的准备
--创建数据库表
create table areas(
aid int primary key,
atitle varchar(20),
pid int
);
--从sql文件中导入数据
-- source 具体地址/areas.sql;
source areas.sql;

--查询一共有多少个省
select * from areas where pid is null;

--例1：查询省的名称为“山西省”的所有城市
select aid from areas where atitle = ‘山西省‘;
select * from areas where pid = (select aid from areas where atitle = ‘山西省‘);

select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle=‘山西省‘;

--例2：查询市的名称为“广州市”的所有区县
select * from areas where pid = (select aid from areas where atitle = ‘广州市‘);

select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle=‘广州市‘;