-- 資料的準備
    -- 建立一個數據庫
    create database python_test charset=utf8;

    -- 使用一個數據庫
    use python_test;

    -- 顯示使用的當前資料是哪個?
    select database();

    -- 建立一個數據表
    -- 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
    );



-- 查詢
    -- 查詢所有欄位
    -- select * from 表名;
    select * from students;
    select * from classes;
    select id, name from classes;

    -- 查詢指定欄位
    -- select 列1,列2,... from 表名;
    select name, age from students;
    
    -- 使用 as 給欄位起別名
    -- select 欄位 as 名字.... from 表名;
 

    select name as 姓名, age as 年齡 from students;

    -- select 表名.欄位 .... from 表名;
    select students.name, students.age from students;

    
    -- 可以通過 as 給表起別名
    -- select 別名.欄位 .... from 表名 as 別名;
    select students.name, students.age from students;
    select s.name, s.age from students as s;
    -- 失敗的select students.name, students.age from students as s;


    -- 消除重複行
    -- distinct 欄位
    select distinct gender from students;


-- 條件查詢
    -- 比較運算子
        -- select .... from 表名 where .....
        -- >
        -- 查詢大於18歲的資訊
        select * from students where age>18;
        select id,name,gender from students where age>18;

        -- <
        -- 查詢小於18歲的資訊
        select * from students where age<18;

        -- >=
        -- <=
        -- 查詢小於或者等於18歲的資訊

        -- =
        -- 查詢年齡為18歲的所有學生的名字
        select * from students where age=18;


        -- != 或者 <>


    -- 邏輯運算子
        -- and
        -- 18到28之間的所以學生資訊
        select * from students where age>18 and age<28;
        -- 失敗select * from students where age>18 and <28;


        -- 18歲以上的女性
        select * from students where age>18 and gender="女";
        select * from students where age>18 and gender=2;


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


        -- not
        -- 不在 18歲以上的女性 這個範圍內的資訊
        -- select * from students where not age>18 and gender=2;
        select * from students where not (age>18 and gender=2);

        -- 年齡不是小於或者等於18 並且是女性
        select * from students where (not age<=18) and gender=2;


    -- 模糊查詢
        -- like 
        -- % 替換1個或者多個
        -- _ 替換1個
        -- 查詢姓名中 以 "小" 開始的名字
        select name from students where name="小";
        select name from students where name like "小%";

        -- 查詢姓名中 有 "小" 所有的名字
        select name from students where name like "%小%";

        -- 查詢有2個字的名字
        select name from students where name like "__";

        -- 查詢有3個字的名字
        select name from students where name like "__";

        -- 查詢至少有2個字的名字
        select name from students where name like "__%";


        -- rlike 正則
        -- 查詢以 周開始的姓名
        select name from students where name rlike "^周.*";

        -- 查詢以 周開始、倫結尾的姓名
        select name from students where name rlike "^周.*倫$";


    -- 範圍查詢
        -- in (1, 3, 8)表示在一個非連續的範圍內
        -- 查詢 年齡為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 不非連續的範圍之內
        -- 年齡不是 18、34歲之間的資訊
        select name,age from students where age not in (12, 18, 34);


        -- between ... and ...表示在一個連續的範圍內
        -- 查詢 年齡在18到34之間的的資訊
        select name, age from students where age between 18 and 34;

        
        -- not between ... and ...表示不在一個連續的範圍內
        -- 查詢 年齡不在在18到34之間的的資訊
        select * from students where age not between 18 and 34;
        select * from students where not age between 18 and 34;
        -- 失敗的select * from students where age not (between 18 and 34);


    -- 空判斷
        -- 判空is null
        -- 查詢身高為空的資訊
        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 欄位
    -- asc從小到大排列，即升序
    -- desc從大到小排序，即降序

    -- 查詢年齡在18到34歲之間的男性，按照年齡從小到到排序
    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;


    -- 查詢年齡在18到34歲之間的女性，身高從高到矮排序
    select * from students where (age between 18 and 34) and gender=2 order by height desc;
    

    -- order by 多個欄位
    -- 查詢年齡在18到34歲之間的女性，身高從高到矮排序, 如果身高相同的情況下按照年齡從小到大排序
    select * from students where (age between 18 and 34) and gender=2 order by height desc,id desc;


    -- 查詢年齡在18到34歲之間的女性，身高從高到矮排序, 如果身高相同的情況下按照年齡從小到大排序,
    -- 如果年齡也相同那麼按照id從大到小排序
    select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc,id desc;

    
    -- 按照年齡從小到大、身高從高到矮的排序
    select * from students order by age asc, height desc;


-- 聚合函式
    -- 總數
    -- count
    -- 查詢男性有多少人，女性有多少人
    select * from students where gender=1;
    select count(*) from students where gender=1;
    select count(*) as 男性人數 from students where gender=1;
    select count(*) as 女性人數 from students where gender=2;


    -- 最大值
    -- max
    -- 查詢最大的年齡
    select age from students;
    select max(age) from students;

    -- 查詢女性的最高 身高
    select max(height) from students where gender=2;

    -- 最小值
    -- min

    
    -- 求和
    -- sum
    -- 計算所有人的年齡總和
    select sum(age) from students;

    
    -- 平均值
    -- avg
    -- 計算平均年齡
    select avg(age) from students;


    -- 計算平均年齡 sum(age)/count(*)
    select sum(age)/count(*) from students;


    -- 四捨五入 round(123.23 , 1) 保留1位小數
    -- 計算所有人的平均年齡，保留2位小數
    select round(sum(age)/count(*), 2) from students;
    select round(sum(age)/count(*), 3) from students;

    -- 計算男性的平均身高 保留2位小數
    select round(avg(height), 2) from students where gender=1;
    -- select name, round(avg(height), 2) from students where gender=1;

-- 分組

    -- group by
    -- 按照性別分組,查詢所有的性別
    select name from students group by gender;
    select * from students group by gender;
    select gender from students group by gender;
    -- 失敗select * from students group by gender;

    -- 計算每種性別中的人數
    select gender,count(*) from students group by gender;


    -- 計算男性的人數
    select gender,count(*) from students where gender=1 group by gender;


    -- group_concat(...)
    -- 查詢同種性別中的姓名
     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
    -- 查詢平均年齡超過30歲的性別，以及姓名 having avg(age) > 30
    select gender, group_concat(name),avg(age) from students group by gender having avg(age)>30;
    
    -- 查詢每種性別中的人數多於2個的資訊
    select gender, group_concat(name) from students group by gender having count(*)>2;



-- 分頁
    -- limit start, count

    -- 限制查詢出來的資料個數
    select * from students where gender=1 limit 2;

    -- 查詢前5個數據
    select * from students limit 0, 5;

    -- 查詢id6-10（包含）的書序
    select * from students limit 5, 5;


    -- 每頁顯示2個，第1個頁面
    select * from students limit 0,2;

    -- 每頁顯示2個，第2個頁面
    select * from students limit 2,2;

    -- 每頁顯示2個，第3個頁面
    select * from students limit 4,2;

    -- 每頁顯示2個，第4個頁面
    select * from students limit 6,2; -- -----> limit (第N頁-1)*每個的個數, 每頁的個數;

    -- 每頁顯示2個，顯示第6頁的資訊, 按照年齡從小到大排序
    -- 失敗select * from students limit 2*(6-1),2;
    -- 失敗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;



-- 連線查詢
    -- inner join ... on

    -- select ... from 表A inner join 表B;
    select * from students inner join classes;

    -- 查詢 有能夠對應班級的學生以及班級資訊
    select * from students inner join classes on students.cls_id=classes.id;

    -- 按照要求顯示姓名、班級
    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;

    -- 給資料表起名字
    select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;

    -- 查詢 有能夠對應班級的學生以及班級資訊，顯示學生的所有資訊，只顯示班級名稱
    select s.*, c.name from students as s inner join classes as c on s.cls_id=c.id;
    
    -- 在以上的查詢中，將班級姓名顯示在第1列
    select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id;

    -- 查詢 有能夠對應班級的學生以及班級資訊, 按照班級進行排序
    -- 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;

    -- 當時同一個班級的時候，按照學生的id進行從小到大排序
    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
    -- 查詢每位學生對應的班級資訊
    select * from students as s left join classes as c on s.cls_id=c.id;

    -- 查詢沒有對應班級資訊的學生
    -- 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
    -- 將資料表名字互換位置，用left join完成

-- 自關聯
    -- 省級聯動 url:http://demo.lanrenzhijia.com/2014/city0605/

    -- 查詢所有省份
    select * from areas where pid is null;

    -- 查詢出山東省有哪些市
    select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle="山東省";
    select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle="山東省";

    -- 查詢出青島市有哪些縣城
    select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle="青島市";
    select * from areas where pid=(select aid from areas where atitle="青島市")


-- 子查詢
    -- 標量子查詢
    -- 查詢出高於平均身高的資訊

    -- 查詢最高的男生資訊
    select * from students where height = 188;
    select * from students where height = (select max(height) from students);

    -- 列級子查詢
    -- 查詢學生的班級號能夠對應的學生資訊
    -- select * from students where cls_id in (select id from classes);