– 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庫是哪個?
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 classes;

-- 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 classes;

-- 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歲之間所有學生的資訊
    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歲以上或者身高查過180(包含)以上
    select * from students where age>18 and 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.*";

    -- 查詢已 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, age asc;

-- 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動大到小排序
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 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格式
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 age desc limit 0,2;

– 連結查詢 – 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 又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 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
-- 將資料表名字互換位置，用left join完成

-- 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 * from areas as city inner join areas as province on city.pid=province.aid where province.atitle="青島市";

– 子查詢 – 標量子查詢 – 查詢出高於平均身高的資訊 select * from students where height > (select avg(height) from students);

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