MySQL-查询

Posted 2020-12-02 阴天气球

-- 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是哪个?
select database();

-- 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
);

 

insert into students (name,age) values("赵日天",9);

insert into students set name = "李杀神",age=10;
-- 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 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 as 年龄 from students;

-- 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;

-- 消除重复行
-- distinct 字段
select distinct 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;

-- >=
-- <=
-- 查询小于或者等于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";
select * from students where age>18 and gender=2;

-- or
-- 18以上或者身高查过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=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.*";

-- 查询以 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的姓名
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;

-- 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,id desc;

-- 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从大到小排序
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 name, 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;

-- 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个数
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

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

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

 

-- 连接查询
-- 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 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，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.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 * from students as s left join classes as c on s.cls_id=c.id having c.id is null;
select * from students as s left join classes as c on s.cls_id=c.id where c.id is null;

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

 

-- 自关联

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

-- 子查询
-- 标量子查询
-- 查询出高于平均身高的信息

-- 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 (select id from classes);