一)內連接(等值連接)：查詢客戶姓名，訂單編號，訂單價格

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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：內連接(等值連接)只能查詢出多張表中，連接字段相同的記錄

二)外連接：按客戶分組，查詢每個客戶的姓名和訂單數

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左外連接：

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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右外連接：

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：外連接既能查詢出多張表中，連接字段相同的記錄；又能根據一方，將另一方不符合相同記錄強行查詢出來

三)自連接：求出AA的老板是EE --------------------------------------------------- 內自連接： select users.ename,boss.ename from emps users inner join emps boss on users.mgr = boss.empno; --------------------------------------------------- 外自連接： select users.ename,boss.ename from emps users left outer join emps boss on users.mgr = boss.empno; --------------------------------------------------- 注意：自連接是將一張表，通過別名的方式，看作多張表后，再進行連接。 這時的連接即可以采用內連接，又可以采用外連接