不好意思，我一心只想学习！
--数据库操作前的准备
-- 创建数据库
-- create database python_test_1 charset=utf8;
-- 使用数据库
-- use python_test_1;
-- 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
-- );
-- 查询练习
-- 查询所有字段
-- select * from 表名;
select * from students;
-- 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 name2 from students;
-- select 表名.字段 .... from 表名;
select students.name from students;
-- 可以通过 as 给表起别名
-- select 别名.字段 .... from 表名 as 别名;
select name from students as s;
select s.name from students as s;
-- 消除重复行(查性别)
-- distinct 字段
select distinct gender from students;
# 分组这个用的比较多
select gender from students group by gender;
-- 条件查询
-- 比较运算符
-- select .... from 表名 where .....
-- >
-- 查询年纪大于18岁的信息
select * from students where age > 18;
-- <
-- 查询年纪小于18岁的信息
select * from students where age < 18;
-- >=
-- <=
-- 查询小于或者等于18岁的信息
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 <>
-- 查询年龄不为18岁的所有学生的名字
select * from students where age != 18;
select * from students where age <> 18;
-- 逻辑运算符
-- and
-- 18和28之间的所以学生信息
select * from students where age > 18 and age < 28;
-- 18岁以上的女性
select * from students where age > 18 and gender = "女";
-- or
-- 18以上或者身高高过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 = "女");
select * from students where not age > 18 and gender = "女";
# 以后如果工作中对语句有疑问的可以直接问领导
-- 模糊查询(where name like 要查询的数据)
-- like
-- % 替换任意个
-- _ 替换1个
-- 查询姓名中 以 "小" 开始的名字
select * from students where name like '小%';
-- 查询姓名中 有 "小" 所有的名字
select * from students where name like '%小%';
-- 查询有2个字的名字
select * from students where name like '__';
-- 查询有3个字的名字
select * from students where name like '___';
-- 查询至少有2个字的名字
select * from students where name like "__%";
-- 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或34的姓名
select * from students where age =18 or age = 34 or age = 56;
select * from students where age in(18,34);
-- not in 不非连续的范围之内
-- 年龄不是 18或34岁的信息
select * 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之间的的信息
select * from students where age >18 and age < 34;
select * from students where age between 18 and 34; # 包含两端的数据
-- not between ... and ...表示不在一个连续的范围内
-- 查询 年龄不在18到34之间的的信息
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;
-- 判非空is not null
select * from students where height is not null;
-- 排序
-- order by 字段
-- asc
-- asc从小到大排列，即升序
-- desc
-- desc从大到小排序，即降序
-- 查询年龄在18到34岁之间的男性，按照年龄从小到大到排序
select * from students where age > 18 and age < 34 and gender = "男" 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 = '女' 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 ='女' 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 ='女' order by height desc,age asc,id desc;
-- 聚合函数
-- 总数
-- count
-- 查询男性有多少人
select count(*) from students ;
-- 最大值
-- max
-- 查询最大的年龄
select max(age) from students;
-- 查询女性的最高 身高
select max(height) from students where gender = '女';
-- 最小值
-- min
select min(age) from students;
-- 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 avg(height) from students;
# 聚合函数计算的时候不会把null计算进去
-- 分组
# 分组的口诀
select 分组的字段 from 表名 group by 分组字段;
-- group by
-- 按照性别分组,查询所有的性别
select gender from students group by gender;
# 每种 每类
-- 计算每种性别中的人数
select gender,count(*) from students 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;
-- 查询每组性别的平均年龄
select gender,avg(age) from students group by gender;
-- having(注意having和group by 连用 having后通常也要跟 聚合函数)
-- 查询平均年龄超过30岁的性别，以及姓名
select gender,avg(age),group_concat(name) 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,count(*) from students group by gender having count(*) > 2;
-- with rollup 汇总的作用(了解)
--select gender,count(*) from students group by gender with rollup;
select gender ,count(*) from students group by gender with rollup having count(*) >2;
-- 分页
-- limit start, count
-- limit 放在最后面(注意)
# 起始位置= (页数-1) * 每一页的个数
-- 限制查询出来的数据个数
-- 查询前5个数据
select * from students limit 0,5;
-- 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;
-- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
select * from students order by age asc limit 10,2;
select * from students where
select 分组 from students group by 分组字段 having 条件查询
select * from 表a inner join 表b on 连接的条件
-- 连接查询
-- inner join ... on
-- select ... from 表A inner join 表B;
-- 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;
select students.name as s_name,classes.name as c_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 students.*，只显示班级名称 classes.name.
select students.* , classes.name from students inner join classes on students.cls_id = classes.id;
-- 在以上的查询中，将班级名显示在第1列
select classes.name,students.* from students inner join classes on students.cls_id = classes.id;
-- 查询 有能够对应班级的学生以及班级信息, 按照班级名进行排序
select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc;
-- 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 classes.name,students.* from students inner join classes on students.cls_id = classes.id
order by classes.name asc,students.id asc;
左连接:左边的表不管在右边的表中有没有找到数据都显示
-- left join
-- 查询每位学生对应的班级信息
select * from students left join classes on students.cls_id = classes.id;
-- 查询没有对应班级信息的学生
select * from students left join classes on students.cls_id = classes.id where classes.name is null;
# 右连接,右边的表不管在左边有没有找到ovrnn都显示
-- right join on
-- 将数据表名字互换位置，用left join完成
select * from students right join classes on students.cls_id = classes.id;
-- 子查询
-- 标量子查询: 子查询返回的结果是一个数据(一行一列)
-- 列子查询: 返回的结果是一列(一列多行)
-- 行子查询: 返回的结果是一行(一行多列)
-- 查询出高于平均身高的信息(height)
select avg(height) from students;
select * from students where height > 172.076923;
select * from students where height > (select avg(height) from students);
-- 查询学生的班级号能够对应的 学生名字
select * from students where cls_id in(1,2);
select id from classes;
select * from students where cls_id in(select id from classes);
# 自联
--数据操作前的准备
--创建数据库表
create table areas(
aid int primary key,
atitle varchar(20),
pid int
);
--从sql文件中导入数据
-- source 具体地址/areas.sql;
source areas.sql;
--查询一共有多少个省
select * from areas where pid is null;
--例1：查询省的名称为“山西省”的所有城市
select aid from areas where atitle = '山西省';
select * from areas where pid = (select aid from areas where atitle = '山西省');
select * from areas where pid = (select aid from areas where atitle = '浙江省');
select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle = '山西省';
--例2：查询市的名称为“广州市”的所有区县
select * from areas where pid = (select aid from areas where atitle = '广州市');
select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle = '广州市';
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