—— 數據准備
—— 創建一個數據庫
create database python_test charset=utf8;
—— 使用一個數據庫
use python_test;
—— 顯示使用的當前數據是哪個？
select database();
—— 創建一個數據表
—— students表
create table students(
id int primary key not null auto_increment,
name varchar(20) default ‘’,
age tinyint unsigned default 0,
height decimal(5,2),
gender enum(‘男’,’女’,’中性’,’包名’) default ‘保密’,
cls_id int unsigned default 0,
is_delete bit default 0
);
—— classes表
create table classes(
id int primary key auto_increment not null,
name varchar(30) not null
);
— 查詢
— 查詢所有字段
—— select * from 表名;
select * from students;
select * from classes;
select id, name from students;
—— 查詢指定字段
select 字段 … from 表名;
select name, age from students;
—— 使用as給字段起別名 查詢指定字段
select 字段 as 別名… from 表名;
select name as 姓名, age as 年齡 from students;
—— select 表名.字段 … from 表名;
select students.name, students.age from students;
—— 可以通過as 給表起別名
select 別名.字段 。。。 From 表名 as 別名; 表起別名后 只能用別名
select s.name, s.age from students as s;
—— 消除重復行
distinct 字段
select distinct genter from students;
—— 條件查詢
—— 比較運算符
select … from 表名 where …
—>
— 查詢大於18歲的信息
select * from students where age>18;
select id,name,age students where age>18;
— <
查詢小於18歲的信息
select * from students where age<18;
— >=
— <= 查詢小於或大於等於18歲的信息
select * from students where age>=18;
select * from students where age<=18;
— = 查詢年齡為18歲的所有學生的名字
select * from students where age=18;
— != 或者 <>
select * from students where age!=18;
select * from students where age<>18;
—— 邏輯運算符
—— and
—— 18到28之間的所有學生信息
select * from students where age>18 and age<28;
—— 18以上的女性信息
select * from students where age>18 and gender=2
—— or
—— 18以上或者身高超過180(包含)以上
select * from students age>18 or height>=180;
—— not
—— 不在18以上的女性 這個范圍內的信息
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 like '小%';
—- 查詢姓名中 包含‘小’所有的名字
select name from students where name like ‘%小%’;
— 查詢有2個字的姓名
select name from students where name like '__';
— 查詢有3個字的姓名
select name from students where name like '__';
—- 查詢至少有3個字以上的姓名
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￥’;
—— 范圍查詢
— in （1, 3, 8）表示在一個非連續的范圍內
— 查詢年齡為18、34的姓名
select * from students where age=18 and age=34;
select name, age from students where age in(18, 34);
— not in 不非連續的范圍之內
— 年齡不是18、34 歲之間的信息
select name, age from students where age not in(18, 34);
— between … and … 表示在一個連續的范圍內
— 查詢年齡在18 到34 之間的信息
select * from students where age between 18 and 34;
— not between … and … 表示不再一個連續的范圍內
— 查詢 年齡不再18到34之間的信息
select * from students where age not between 18 and 34;
select * from students where not age between 18 and 34;
—— 空判斷
— 判空 is null
— 查詢身高為空的信息
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 從小到大排序 即升序
— desc 從大到小排序 即降序
— 查詢年齡在10到60之間的男性， 按照年齡從小到大排序
select * from students where (age between 10 and 60) and gender=1 order by age;
select * from students where (age between 10 and 60) 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 多個字段
— 查詢年齡在18到34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) order by height desc, age asc;
— 如果年齡也相同 那么按照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;
—— 聚合函數
— 總數
— count
— 查詢男性有多少人，女性有多少人
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 max(age) as 年齡最大是 from studentes;
— 查詢女性的最高 身高
select max(height) as 女性的最高身高是 from students where gender=2;
— 最小值
— min
select min(height) from students where gender=2;
— 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;
— 四舍五入 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(avg(age), 2) 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;
—— 分組
— group by
— 按照性別分組，查詢所有的性別
select gender 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;
- 查詢男性姓名，id，age
select gender, group_concat(name, id, age) from students where gender=1 group by gender
select gender, group_concat("姓名：",name, "id:",id, "年齡：",age) 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 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個信息
select gender, group_concat(name) from students group by gender having count(*)>2;
—— 分頁
— limit start, count
— 限制查詢出來的數據個數
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，第一個頁面
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，第三個頁面
select * from students limit 4, 2;
— 每頁顯示2個，第四個頁面
select * from students limit 6, 2; ——> limit (第n頁-1)*每頁的個數, 每頁的個數;
— 每頁顯示2個，顯示第6頁信息，按照年齡從小到大排序
select * from students order by age limit 10, 2;
—— 連接查詢
— 內連接 inner join … on
— select * from 表1 inner join 表2 on 條件;
— 查詢 有能夠對應班級的學生以及班級信息
select * from students inner join classes on students.cls_id=classes.id;
— 按照要求顯示姓名、班級
select students.name, classes.name from students inner join classes on students.cls_id=classes.id;
— 給數據表起名字
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信息，顯示學生的所有信息，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信息，按照班級進行排序
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;
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信息
select * from students as s left join classes as c on s.cls_id=c.id;
— 查詢沒有對應班級信息的學生
select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;
— right join on
— 將數據表名互換位置，用left join完成
—— 自關聯
— 省級聯動
— 查詢所有省份
select * from area_info where pid in null;
— 查詢出山東省有哪些城市
select * from area_info as province inner join area_info as city on province.aid=city.pid having province.atitle='山東省';
—- 查出青島市有哪些縣城
select * from area_info as city inner join area_info as country on city.aid= country.pid having city.atitle=‘青島市’;
—— 子查詢
—- 標量子查詢
— 查詢出高於平均身高的信息
select * from students where height>(select avg(height) from students);