-- 创建数据库
create database if not exists schoool default charset=utf8;
-- 使用数据库
use schoool;
-- 创建表格
create table studentinfo(
    id int primary key auto_increment,
    `name` varchar(20),
    sex char,
    age int ,
    address varchar(100)
);
-- 查看数据表
show tables;
-- 插入数据
insert into studentinfo (`name` ,sex ,age ,address) values ("贾宝玉","男",18,"北平"),
("贾政","男",40,"浙江"),("晴雯","女",20,"四川"),("袭人","女",29,"贵州"),
("薛宝钗","女",19,"北平"),("林黛玉","女",17,"浙江");

-- 查看数据 通配符* 表示所有
select * from studentinfo;
-- 手动写出字段，全量查询
select id,`name`,sex,age,address from studentinfo;
-- 只查询指定的数据列
select id,`name`,sex from studentinfo;
-- 条件查询
select * from studentinfo where sex='女';
-- 根据查询结果排序 order by 
select * from studentinfo order by age desc;
select * from studentinfo order by age desc,id desc;
-- 显示查询结果的条数
select * from studentinfo limit 3;
-- 显示查询结果中从索引为5开始的往后3条数据
select * from studentinfo limit 5, 3;
-- 复合
select * from studentinfo where sex='女' order by age desc limit 3;
-- 别名 AS as 可以省略
select id as 编号 ,`name` as 姓名,sex as 性别 ,age as 年龄 ,address as 地址 from studentinfo; 
select id  编号 ,`name`  姓名,sex  性别 ,age  年龄 ,address  地址 from studentinfo; 
-- 单条件查询
select * from studentinfo where age <20;
select * from studentinfo where age is not null;
-- 多条件查询
-- 不等于
select * from studentinfo where age!=20;
select * from studentinfo where age<>20;
select * from studentinfo where not (age=20);
-- 与 或
select * from studentinfo where age >20 && sex='女';
select * from studentinfo where age >20 and sex='女';
select * from studentinfo where age >20 || sex='女';
select * from studentinfo where age >20 or sex='女';
-- 去重 去除重复的数据
select distinct `name`,sex,age,address from studentinfo;