SQL 连接 JOIN 例解。（左连接，右连接，全连接，内连接，交叉连接，自连接）

很易懂的例子解说
http://www.cnblogs.com/eflylab/archive/2007/06/25/794278.html

右连接,等号右侧的所有记录都会显示,无论在左侧是否得到匹配


关于左连接、右连接、外接连总结
在9i以前可以这么写:
左联：

select a.id,a.name,b.address from a,b  
where a.id=b.id(+)

右联:

select a.id,a.name,b.address from a,b  
where a.id(+)=b.id

外联

SELECT a.id,a.name,b.address
FROM a,b
WHERE a.id = b.id(+)
UNION
SELECT b.id,'' name,b.address
FROM b
WHERE NOT EXISTS (
SELECT * FROM a
WHERE a.id = b.id);

在9i以上，已经开始支持SQL99标准，所以，以上语句可以写成：
默认内部联结：

select a.id,a.name,b.address,c.subject
from (a inner join b on a.id=b.id)  
inner join c on b.name = c.name
where other_clause

左联

select a.id,a.name,b.address
from a left outer join b on a.id=b.id  
where other_clause

右联

select a.id,a.name,b.address
from a right outer join b on a.id=b.id  
where other_clause

外联

select a.id,a.name,b.address
from a full outer join b on a.id=b.id  
where other_clause
or
select a.id,a.name,b.address
from a full outer join b using (id)
where other_clause