一）内连接（等值连接）：查询客户姓名，订单编号，订单价格
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select c.name,o.isbn,o.price
from customers c inner join orders o
where c.id = o.customers_id;
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select c.name,o.isbn,o.price
from customers c join orders o
where c.id = o.customers_id;
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select c.name,o.isbn,o.price
from customers c,orders o
where c.id = o.customers_id;
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select c.name,o.isbn,o.price
from customers c join orders o
on c.id = o.customers_id;
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注意：内连接（等值连接）只能查询出多张表中，连接字段相同的记录
二）外连接：按客户分组，查询每个客户的姓名和订单数
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左外连接：
select c.name,count(o.isbn)
from customers c left outer join orders o
on c.id = o.customers_id
group by c.name;
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右外连接：
select c.name,count(o.isbn)
from orders o right outer join customers c
on c.id = o.customers_id
group by c.name;
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注意：外连接既能查询出多张表中，连接字段相同的记录；又能根据一方，将另一方不符合相同记录强行查询出来
三）自连接：求出AA的老板是EE
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内自连接：
select users.ename,boss.ename
from emps users inner join emps boss
on users.mgr = boss.empno;
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外自连接：
select users.ename,boss.ename
from emps users left outer join emps boss
on users.mgr = boss.empno;
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注意：自连接是将一张表，通过别名的方式，看作多张表后，再进行连接。
这时的连接即可以采用内连接，又可以采用外连接