7.md

# 创建一个user表
create table users(
    id int primary key auto_increment,
    name varchar(20),
    dpt_id int
);
# 向user表中插入数据
insert into users values
    (null,'刘备',1),
    (null,'关羽',2),
    (null,'赵飞',2),
    (null,'吕布',2),
    (null,'小乔',3),
    (null,'大乔',3);
 
# 创建一个department表
create table department(
    id int primary key auto_increment,
    name varchar(20)
);
 
# 向department表中插入数据
insert into department values
    (null,'指挥部'),
    (null,'作战部'),
    (null,'后勤部');
 
# 查询 users 表中的第三和第四条数据 (第一个数字是偏移量，第二个数字是查询几条)
select * from users limit 2,2;
 
# 为users表取别名
select u.name from users as u;
 
# 为users表中的字段取别名
select name as '姓名', dpt_id as '部门' from users;
 
# 表的关联查询 查询users表中员工的姓名和所在的部门
select u.name as '姓名',d.name as '部门' from users as u inner join department as d where u.dpt_id = d.id;
 
# 表的关联查询 查询users表中吕布所在的部门 (on 可以换成 where)
select d.name from department as d inner join users as u on u.dpt_id=d.id and u.name='吕布';