create database if not exists school default charset=utf8;

use school;

create table if not exists studentinfo(
    id int primary key auto_increment,
    `name` varchar(20),
    sex char,
    age int,
    address varchar(100)
);

show tables;

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;
-- 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';

-- distinct 去除重复的数据
select distinct `name`, sex, age, address from studentinfo;