join 使用详解方式
（1) cross join
参与select语句所有表的的所有行的笛卡尔乘积
select au_lname ,title
from authors cross join titiles
outer join 对参与join的两个表有主从之分，处理方式以主表的每条数据去match 从属表的列，合乎条件的数据是我们所要的答案，不合乎条件的也是我们要的答案，只不过哪些从属表选取的列将被添上null。
(2) left join
左边的为主表，右边为从属表
select a.cust_id ,b.order_date,b.tot_ant
from customer a left join sales b
on (a.cust_id =b.cust_id and b.order_date>''1996/10/15'')
可以写为
select a.cust_id,b.order_date,b.tot_ant
from custom a
left join (select * from sales where order_date>''1996/10/15'') b
on a.cust_id =b.cust_id
(3) right join
左边的表为从属表，右边的表为主表
(4) self join
self join 常用在同一表内不同数据间对同一列的比较
select a.emp_no,a.emp_name,b.emp_no,b.emp_name,a.date_hired
from employee a
join employee b
on (a.emp_no!=b.emp_no and a.date_hired=b.date_hired)
order by a.date_hired
这样会重复数据，只要加上一句 and a.emp_name>b.emp_name
(5) full join
不仅列出符合条件的数据，两边未符合join条件的数据也会一并列出。哪些未符合join条件的数据如果在select列中无法得到对应的值则填上null
select a.cust_id,b.tot_amt
from customer a full join sales b
on a.cust_id=b.cust_id
有表
id ic name amount
I * *
c
i
c
i
i
要求结果为
ic name amount ic name amount
i c
i c
i
i
select aaa.*,bbb.*
from ( select (select count(id) from aa as b where (b.id<a.id) and (ic=''i'')) as newid, * from aa a where ic=''i'') aaa
full join
(select (select count(id) from aa as b where b.id<a.id and ic=''c'') as newid,* from
aa a where ic=''c'') bbb
on aaa.newid=bbb.newid
order by aaa.name
6.使用 HASH 和 MERGE 联接提示
此示例在 authors、titleauthors 和 titles 表之间建立三表联接，以生成一个作者及其著作的列表。查询优化器使用 MERGE 联接将 authors 和 titleauthors (A x TA) 联接在一起。然后，将 authors 和 titleauthors MERGE 联接 (A x TA) 的结果与 titles 表进行 HASH 联结以生成 (A x TA) x T。
重要 指定联接提示后，要执行 INNER JOIN 时 INNER 关键字不再为可选，而必须显式说明。
USE pubs
SELECT SUBSTRING((RTRIM(a.au_fname) + '' '' + LTRIM(a.au_lname)), 1, 25)
AS Name, SUBSTRING(t.title, 1, 20) AS Title
FROM authors a INNER MERGE JOIN titleauthor ta
ON a.au_id = ta.au_id INNER HASH JOIN titles t
ON t.title_id = ta.title_id
ORDER BY au_lname ASC, au_fname ASC
下面是结果集：
Warning: The join order has been enforced because a local join hint is used.
Name Title
------------------------- --------------------
Abraham Bennet The Busy Executive''s
Reginald Blotchet-Halls Fifty Years in Bucki
Cheryl Carson But Is It User Frien
Michel DeFrance The Gourmet Microwav
Innes del Castillo Silicon Valley Gastr
... ...
Johnson White Prolonged Data Depri
Akiko Yokomoto Sushi, Anyone?
(25 row(s) affected)
