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 --------------------------------------------------- 内自连接： select users.ename,boss.ename from emps users inner join emps boss on users.mgr = boss.empno; --------------------------------------------------- 外自连接： select users.ename,boss.ename from emps users left outer join emps boss on users.mgr = boss.empno; --------------------------------------------------- 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