1、使用表别名
例子：
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 = 'RGAN01';
/*
cust_name, cust_contact
Fun4All Denise L. Stephens
The Toy Store Kim Howard
*/
说明：表别名只在查询中使用，不返回到客户端。
2、使用不同类型的联结
2、1自联结
例子：给Jim Jones公司的所有顾客发信件
方法一：子查询
select cust_id, cust_name, cust_contact
from customers
where cust_name = (select cust_name
from customers
where cust_contact = 'Jim Jones');
方法二：自联结
select C1.cust_id, C1.cust_name, C1.cust_contact
from customers as C1, customers as C2
where C1.cust_name = C2.cust_name
and C2.cust_contact = 'Jim Jones';
/*
cust_id cust_name cust_contact
1000000003 Fun4All Jim Jones
1000000004 Fun4All Denise L. Stephens
*/
说明:一般DBMS处理联结比子查询快，Oracle 没有as，取别名直接 customers C.
2、2自然联结
自然联结排除多次出现，使每一列只返回一次，自然联结要求你只能选择那些唯一的列，
一般通过对一个表使用通配符（*），而对其他表的列使用明确的子集来完成。
例子：
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 = 'RGAN01';
/*
1000000004 Fun4All 829 Riverside Drive Phoenix AZ 88888 USA
1000000005 The Toy Store 4545 53rd Street Chicago IL 54545 USA
*/
2、3外联结
左外联结
select customers.cust_id, orders.order_num
from customers left outer join orders
on customers.cust_id = orders.cust_id;
/*cust_id order_num
1000000001 20005
1000000003 20006
1000000004 20007
1000000005 20008
1000000001 20009
1000000002
*/
右外联结
select customers.cust_id, orders.order_num
from customers right outer join orders
on customers.cust_id = orders.cust_id;
/*
cust_id order_num
1000000001 20005
1000000003 20006
1000000004 20007
1000000005 20008
1000000001 20009
*/
2、4使用带聚集函数的联结
例子：查询所有顾客及每个顾客的订单数
select customers.cust_id, count(orders.order_num) as num_ord
from customers inner join orders
on customers.cust_id = orders.cust_id
group by customers.cust_id;
/*
cust_id, num_ord
1000000001 2
1000000003 1
1000000004 1
1000000005 1*/
例子：左外联结所有顾客订单
select customers.cust_id, count(orders.order_num) as num_ord
from customers left outer join orders
on customers.cust_id = orders.cust_id
group by customers.cust_id;
/*
cust_id, num_ord
1000000001 2
1000000002 0
1000000003 1
1000000004 1
1000000005 1
*/