—— 数据准备
—— 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 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
);
— 查询
— 查询所有字段
—— select * from 表名;
select * from students;
select * from classes;
select id, name from students;
—— 查询指定字段
select 字段 … from 表名;
select name, age from students;
—— 使用as给字段起别名 查询指定字段
select 字段 as 别名… 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; 表起别名后 只能用别名
select s.name, s.age from students as s;
—— 消除重复行
distinct 字段
select distinct genter 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,age students where age>18;
— <
查询小于18岁的信息
select * from students where age<18;
— >=
— <= 查询小于或大于等于18岁的信息
select * from students where age>=18;
select * from students where age<=18;
— = 查询年龄为18岁的所有学生的名字
select * from students where age=18;
— != 或者 <>
select * from students where age!=18;
select * from students where age<>18;
—— 逻辑运算符
—— and
—— 18到28之间的所有学生信息
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以上或者身高超过180(包含)以上
select * from students age>18 or height>=180;
—— not
—— 不在18以上的女性 这个范围内的信息
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 like '小%';
—- 查询姓名中 包含‘小’所有的名字
select name from students where name like ‘%小%’;
— 查询有2个字的姓名
select name from students where name like '__';
— 查询有3个字的姓名
select name from students where name like '__';
—- 查询至少有3个字以上的姓名
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 * from students where age=18 and age=34;
select name, age from students where age in(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(18, 34);
— between … and … 表示在一个连续的范围内
— 查询年龄在18 到34 之间的信息
select * from students where age between 18 and 34;
— not between … and … 表示不再一个连续的范围内
— 查询 年龄不再18到34之间的信息
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;
— 判非空 is not null
select * from students where height is not null;
—— 排序
— order by 字段
— asc 从小到大排序 即升序
— desc 从大到小排序 即降序
— 查询年龄在10到60之间的男性， 按照年龄从小到大排序
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) order by height desc, age asc;
— 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 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 max(age) as 年龄最大是 from studentes;
— 查询女性的最高 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
— 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(avg(age), 2) 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;
—— 分组
— group by
— 按照性别分组，查询所有的性别
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;
- 查询男性姓名，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, "年龄：",age) 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 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，第一个页面
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，第三个页面
select * from students limit 4, 2;
— 每页显示2个，第四个页面
select * from students limit 6, 2; ——> limit (第n页-1)*每页的个数, 每页的个数;
— 每页显示2个，显示第6页信息，按照年龄从小到大排序
select * from students order by age limit 10, 2;
—— 连接查询
— 内连接 inner join … on
— select * from 表1 inner join 表2 on 条件;
— 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.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;
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 students as s left join classes as c on s.cls_id=c.id having c.id is null;
— right join on
— 将数据表名互换位置，用left join完成
—— 自关联
— 省级联动
— 查询所有省份
select * from area_info where pid in null;
— 查询出山东省有哪些城市
select * from area_info as province inner join area_info as city on province.aid=city.pid having province.atitle='山东省';
—- 查出青岛市有哪些县城
select * from area_info as city inner join area_info as country on city.aid= country.pid having city.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);