-- 数据的准备

-- 创建一个数据库

create database python_test charset=utf8;

-- 使用一个数据库

use python_test;

--显示使用的当前数据库是哪个?

select databases();

--创建一个数据表

-- 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

-- 查询所有字段

-- select * from 表名;

select * from students;

select * from classes;

select id, name from classes;

-- 查询指定字段

-- select 列1,列2,... from 表名;

select name, age from classes;

-- 使用 as 给字段起别名

-- select 字段 as 名字.... from 表名;

select name as 姓名, age as 年龄 from classes;

-- select 表名.字段 .... from 表名;

select students.name, students.age from students;

-- 可以通过 as 给表起别名

-- select 别名.字段 .... from 表名 as 别名;

select students.name, students.age from students;

select s.name, s.age from students as s;

-- 失败的select students.name, students.age from students as s;

-- 消除重复行

-- distinct 字段

select district gender from students;

-- 条件查询

-- 比较运算符

-- select .... from 表名 where .....

-- >

-- 查询大于18岁的信息

select * from students where age>18;

select id,name,gender from students where age>18;

-- <

-- 查询小于18岁的信息

select * from students where age<18;

-- >=

-- <=

-- 查询小于或等于18岁的信息

-- =

--查询年龄为18岁的所有学生的名字

select * from students where age=18;

-- != 或者 <>

-- 逻辑运算符

-- and

-- 18到28岁之间所有学生的信息

select * from students where age>18 and age<28;

-- 失败select * from students where age>18 and <28;

--18岁以上的女性

select * from students where age>18 and gender="女";

select * from students where age>18 and gender=2;

-- or

--18岁以上或者身高查过180(包含)以上

select * from students where age>18 and 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=2);

-- 年龄不是小于或者等于18 并且是女性

select * from students where (not age<=18) and gender=2;

-- 模糊查询

-- like

-- % 替换1个或者多个

-- _ 替换1个

-- 查询姓名中 以 "小" 开始的名字

select name from students where name="小";

select name from students where name like "小%";

-- 查询姓名中 有"小" 所有的名字

select name from students where name like "%小%";

-- 查询有2个字的名字

select name from students where name like "__";

-- 查询有3个字的名字

select name from students where name like "___";

-- 查询至少有2个字的名字

select name from students where name like "__%";

-- rlike 正则

-- 查询以 周开始的姓名

select name from students where name rlike "^周.*";

-- 查询已 周开始、伦结尾的姓名

select name from students where name rlike "^周.*伦$";

-- 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、34的名字

select name,age from students where age=18 or age=34;

select name,age from students where age=18 or age=34 or age=12;

select name,age from students where age in (12, 18, 34);

-- not in 不非连续的范围之内

-- 年龄不是 18、34岁之间的信息

select name,age from students where age not in (12, 18, 34);

-- between ... and ...表示在一个连续的范围内

-- 查询 年龄在18到34之间的信息

select name,age from students where age between 18 and 34;

-- not between ... and ...表示不在一个连续的范围内

-- 查询 年龄不在在18到34之间的的信息

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

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

-- 失败的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;

select * from students where height is NULL;

select * from students where height is Null;

-- 判非空is not null

select * from students where height is not null;

-- 排序

-- order by 字段

-- asc从小到大排列，即升序

-- desc从大到小排序，即降序

-- 查询年龄在18到34岁之间的男性，按照年龄从小到到排序

select * from students where (age between 18 and 34) and gender=1;

select * from students where (age between 18 and 34) and gender=1 order by age;

select * from students where (age between 18 and 34) and gender=1 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=2 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=2 order by height desc, age asc;

-- 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,

-- 如果年龄也相同那么按照id动大到小排序

select * from students where (age between 18 and 34) and gender=2 order by height desc, age asc, id desc;

-- 按照年龄从小到大、身高从高到矮的排序

select * from students order by age asc, height desc;

-- 聚合函数

-- 总数

-- count

-- 查询男性有多少人，女性有多少人

select * from students where gender=1;

select count() from students where gender=1;

select count() as 男性人数 from students where gender=1;

select count(*) as 女性人数 from students where gender=2;

-- 最大值

-- max

-- 查询最大的年龄

select age from students;

select max(age) from students;

-- 查询女性的最高 身高

select max(height) from students where gender=2;

-- 最小值

-- min

-- 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(sum(age)/count(*), 2) from students;

select round(sum(age)/count(*), 3) 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=1;

-- select round(avg(height), 2) from students where gender=1;

-- 分组

-- group by

-- 按照性别分组, 查询所有的性别

select name from students group by gender;

select * from students group by gender;

select gender from students group by gender;

-- 失败select * from students group by gender;

-- 计算每种性别中的人数

select gender,count(*) from students group by gender;

-- 计算男性的人数

select gender,count(*) from students where gender=1 group by gender;

-- group_concat(...)

-- 查询同种性别中的姓名

select gender, group_concat(name) from students where gender=1 group by gender;

select gender, group_concat(name, age, id) from students where gender=1 group by gender;

select gender, group_concat(name, "_", age, " ", id) from students where gender=1 group by gender;

-- having

-- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30

select gender, group_concat(name), 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, group_concat(name) from students group by gender having count(*)>2;

-- 分页

-- limit start, count

-- 限制查询出来的数据格式

select * from students where gender=1 limit 2;

-- 查询前5个数据

select * from students limit 0,5;

-- 查询id6-10(包含)的书序

select * from students limit 5,5;

-- 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; -- -----> limit (第N页-1)*每个的个数, 每页的个数;

-- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序

-- 失败select * from students limit 2*(6-1),2;

-- 失败select * from students limit 10,2 order by age asc;

select * from students order by age asc limit 10,2;

select * from students where gender=2 order by age desc limit 0,2;

-- 链接查询

-- inner join ... on

-- select * from 表A inner join 表B;

select * from students inner join classes;

-- 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.* classes.name from students inner join classes on students.cls_id=classes.id;

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 又能够对应班级的学生以及班级信息, 显示学生的所有信息, 只显示班级名称

select s.*,c.name from students as s inner join classes as c on s.cls_id=c.id;

-- 在以上的查询中，将班级姓名显示在第1列

select c.name,s.* 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, 按照班级进行排序

-- select c.xxx s.xxx from student as s inner join clssses as c on .... order by ....;

select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

-- 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 c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;

-- left join

-- 查询每位学生对应的班级信息

select * from students as s left join classes as c on s.cls_id=c.id;

-- 查询没有对应班级信息的学生

-- select ... from xxx as s left join xxx as c on..... where .....

-- select ... from xxx as s left join xxx as c on..... having .....

select s.*, c.name from students as s left join classes as c on s.cls_id=c.id where c.name is null;

select s.*, c.name from students as s left join classes as c on s.cls_id=c.id having c.name is null;

-- right join

-- 将数据表名字互换位置，用left join完成

-- 查询所有省份

select * from areas where pid is null;

-- 查询出山东省有哪些市

select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle="山东省";

select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle="山东省";

-- 查询出青岛市有哪些县城

select * from areas as city inner join areas as province on city.pid=province.aid where province.atitle="青岛市";

-- 子查询

-- 标量子查询

-- 查询出高于平均身高的信息

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

-- 查询最高的男生信息

select * from students where height = 188;

select * from students where height = (select max(height) from students);

-- 列级子查询

-- 查询学生的班级号能够对应的学生信息

select * from students where cls_id in (1, 2);

select * from students where cls_id in (select id from classes);