-- create table customers(
-- id int primary key auto_increment,
-- name VARCHAR(20)not null,
-- address VARCHAR(20)not NULL
-- );
-- CREATE table orders(
-- order_namre VARCHAR(20) primary key,
-- num char(20) not NULL,
-- price int not null,
-- customers_id int,
-- constraint cus_ord_fk FOREIGN key(customers_id) REFERENCES customers(id)
-- )
自己插入数据即可。
语法:

select * from customers,orders where customers.id=orders.customers_id;

select * from customers c INNER JOIN orders o ON c.id=o.customer_id;

select * from customers as c,orders o where c.id=o.customers_id;
SELECT * from customers as c inner join orders o on c.id=o.customers_id;

select * from customers c LEFT JOIN orders o ON c.id=o.customer_id;

select * from orders o RIGHT JOIN customers c ON c.id=o.customer_id;

select * from students where id in(select s_id from teacher_student where t_id=1);

create table teacher1(
id int primary key auto_increment,
name char(20) not NULL,
subject char(20) not null
);

create table student1(
id int primary key auto_increment,
name char(20) unique not null,
age int null
);

create table tea_stu(
id int PRIMARY KEY,
name char(20),
t_id int,
s_id int,
score int not null,
constraint teacher1_id_fk foreign key(t_id) references teacher1(id),
constraint student_id_fk foreign key(s_id) references student1(id)
);

select s_id from tea_stu where t_id=1;
select * from student1 where id in(2,3)；

select * from student1 where id in(select s_id from tea_stu where t_id=1);

select * from tea_stu as a where a.score>(select avg(b.score) from tea_stu as b where a.s_id=b.s_id);