1、联结表
联结表是利用SQL的select执行的重要操作。
可伸缩：能够适应不断增加的工作量而不失败，设计良好的数据库或者应用程序称为可伸缩性好。
联结是一种机制，用一条select语句关联表，可以联结多个表返回一组输出。
2、创建联结表
select vend_name, prod_name, prod_price
from vendors, products
where vendors.vend_id = products.vend_id;
/*
vend_name, prod_name, prod_price
Doll House Inc. Fish bean bag toy 3.49
Doll House Inc. Bird bean bag toy 3.49
Doll House Inc. Rabbit bean bag toy 3.49
Bears R Us 8 inch teddy bear 5.99
Bears R Us 12 inch teddy bear 8.99
Bears R Us 18 inch teddy bear 11.99
Doll House Inc. Raggedy Ann 4.99
Fun and Games King doll 9.49
Fun and Games Queen doll 9.49
*/
分析：
vendors的第一行去匹配products的满足where条件每一行。
3、内联结
select vend_name, prod_name, prod_price
from vendors inner join products
on vendors.vend_id = products.vend_id;
/*
vend_name, prod_name, prod_price
Doll House Inc. Fish bean bag toy 3.49
Doll House Inc. Bird bean bag toy 3.49
Doll House Inc. Rabbit bean bag toy 3.49
Bears R Us 8 inch teddy bear 5.99
Bears R Us 12 inch teddy bear 8.99
Bears R Us 18 inch teddy bear 11.99
Doll House Inc. Raggedy Ann 4.99
Fun and Games King doll 9.49
Fun and Games Queen doll 9.49
*/
分析：效果第一个例子一样
4、联结多个表
例子：找出订单20007的所物品
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 = 20007;
/*
prod_name, vend_name, prod_price, quantity
18 inch teddy bear Bears R Us 11.99 50
Fish bean bag toy Doll House Inc. 3.49 100
Bird bean bag toy Doll House Inc. 3.49 100
Rabbit bean bag toy Doll House Inc. 3.49 100
Raggedy Ann Doll House Inc. 4.99 50
*/
例子：子查询例子改进
select cust_name, cust_contact
from customers, orders, orderitems
where orderitems.order_num = orders.order_num
and orders.cust_id = customers.cust_id
and prod_id = 'RGAN01';
/*
cust_name, cust_contact
Fun4All Denise L. Stephens
The Toy Store Kim Howard
*/