mysql-7 資料檢索（5）

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連接（join）

SQL最強大的功能之一就是能在資料檢索查詢的執行中連接（join）表。連接就是利用SQL的SELECT能執行的最重要的操作。

例子：此例子包含兩個表，一個表是vendors , vendors表包含所有供應商資訊，每個供應商佔一行，每個供應商具有唯一的標識，此標識稱為主鍵，可以是供應商ID或任何其他唯一值。

products表只儲存產品資訊，它除了儲存供應商ID（vendors表的主鍵）外不儲存其他供應商資訊，vendors表的主鍵又叫做products的外鍵，它將vendors表與products表關聯，利用供應商ID能從vnedors表中找出相應供應商詳細資料。

SELECT * FROM products;

SELECT * FROM vendors;

SELECT * FROM orderitems;

 兩個表連接

SELECT vend_name , prod_name , prod_price FROM vendors , products WHERE vendors.`vend_id` = products.`vend_id` ORDER BY vend_name,prod_name;


SELECT vend_name , prod_name , prod_price FROM vendors INNER JOIN products ON vendors.`vend_id` = products.`vend_id`;

 

多個表連接

SELECT prod_name , vend_name ,prod_price , quantity FROM orderitems , products , vendors WHERE products.`vend_id` = vendors.`vend_id` AND orderitems.`prod_id` = products.`prod_id` AND order_num = 20005;

 

 使用表別名

SELECT CONCAT(RTRIM(vend_name),‘(‘,RTRIM(vend_country),‘)‘) AS vend_title FROM vendors ORDER BY vend_name;

 

建立進階連接，使用表別名

SELECT cust_name , cust_contact FROM customers AS c , orders AS o ,orderitems AS oi WHERE c.`cust_id` = o.`cust_id` AND oi.`order_num` = o.`order_num` AND prod_id = ‘TNT2‘;

自連接

SELECT prod_id,prod_name FROM products WHERE vend_id = (SELECT vend_id FROM products WHERE prod_id = ‘DTNTR‘);   自查詢
SELECT p1.prod_id,p1.prod_name FROM products AS p1, products AS p2 WHERE p1.`vend_id`=p2.`vend_id` AND p2.`prod_id`= ‘DTNTR‘;  自連接

自然連接

SELECT c.* , o.order_num ,o.order_date, oi.prod_id, oi.quantity, OI.item_price FROM customers AS c ,orders AS o, orderitems AS oi WHERE c.`cust_id` =o.`cust_id` AND oi.`order_num` = o.`order_num` AND prod_id = ‘FB‘;

 

外部連接

SELECT customers.`cust_id`,orders.`order_num` FROM customers LEFT OUTER JOIN orders ON customers.`cust_id` = orders.`cust_id`;

　　

 

 

SELECT vend_name , prod_name , prod_price FROM vendors , products WHERE vendors.`vend_id` = products.`vend_id` ORDER BY vend_name,prod_name;SELECT vend_name , prod_name , prod_price FROM vendors INNER JOIN products ON vendors.`vend_id` = products.`vend_id`;SELECT prod_name , vend_name ,prod_price , quantity FROM orderitems , products , vendors WHERE products.`vend_id` = vendors.`vend_id` AND orderitems.`prod_id` = products.`prod_id` AND order_num = 20005;SELECT CONCAT(RTRIM(vend_name),‘(‘,RTRIM(vend_country),‘)‘) AS vend_title FROM vendors ORDER BY vend_name;SELECT cust_name , cust_contact FROM customers AS c , orders AS o ,orderitems AS oi WHERE c.`cust_id` = o.`cust_id` AND oi.`order_num` = o.`order_num` AND prod_id = ‘TNT2‘;SELECT prod_id,prod_name FROM products WHERE vend_id = (SELECT vend_id FROM products WHERE prod_id = ‘DTNTR‘);SELECT p1.prod_id,p1.prod_name FROM products AS p1, products AS p2 WHERE p1.`vend_id`=p2.`vend_id` AND p2.`prod_id`= ‘DTNTR‘;SELECT c.* , o.order_num ,o.order_date, oi.prod_id, oi.quantity, OI.item_price FROM customers AS c ,orders AS o, orderitems AS oi WHERE c.`cust_id` =o.`cust_id` AND oi.`order_num` = o.`order_num` AND prod_id = ‘FB‘;SELECT customers.`cust_id`,orders.`order_num` FROM customers LEFT OUTER JOIN orders ON customers.`cust_id` = orders.`cust_id`;

 

mysql-7 資料檢索（5）