关于MySql 4.0.12左联接left outer join的一些问题

1.department表的创建

CREATE TABLE `department` (
`id` int(8) NOT NULL auto_increment,
`department_name` varchar(8) NOT NULL default '',
PRIMARY KEY (`id`)
) TYPE=MyISAM

2.employee表的创建

CREATE TABLE `employee` (
`id` int(8) NOT NULL auto_increment,
`name` varchar(8) NOT NULL default '',
`department_id` int(8) NOT NULL default '0',
PRIMARY KEY (`id`)
) TYPE=MyISAM

3。department表与employee表数据

department表数据如下



employee表数据如下:



4.没有条件限制时
select
d.*,
e.*
from
department d
left outer join
employee e
on (d.id=e.department_id)
查询结果如下:



5。条件限制为d.id=2时select
d.*,
e.*
from
department d
left outer join
employee e
on (d.id=e.department_id)
and d.id=2

查询结果如下:


6。条件限制为d.id=1时
select
d.*,
e.*
from
department d
left outer join
employee e
on (d.id=e.department_id)
and d.id=1

查询结果如下:



7.让我感到疑问的是:左联接的查询会先把左边表(这里为department 表)全部先出来,然后再根据on(....)条件" on (d.id=e.department_id)" 和and 条件"and d.id=1 "去查询记录,然后与左边的表对应起来,而不是说先根据对左边表的限制(d.id=1)选取相应的记录去做左联接.

以上只是针对mysql 4.0.12做的测试,不知在oracle下会是怎样的结果,还有就是mysql中用到左联接时,and 条件不能放到where中,否则不能实现左联接,