MySQL查询

查询：

-- 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
    );



-- 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;
    -- 失败的select students.name, students.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;
        -- 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;
        -- 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,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;
    -- 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
    -- 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 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.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 * 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
    -- 省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/

    -- 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
    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 (select id from classes);