昨天我用MySQL来实现了ORACLE的递归语句CONNECT BY， 看起来稍复杂些。今天来看看POSTGRESQL如何实现ORACLE的CONNECT BY。
还是用昨天同样的表以及数据。POSTGRESQL自诩最像ORACLE的数据库，所以大部分语句也就都可以简单而且变相的实现了。
在这点上可以用他自己带的WITH递归功能，还可以用第三方扩展带来的类似connect by 函数。
先来看第一点，用递归的WITH来展现这棵树的路径。
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t_girl=# with recursive tmp_country(id,path) as
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t_girl-# (
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t_girl(# select a.id,‘/‘||b.name as "path" from country_relation as a inner join country as b on (a.id = b.id) where a.parentid is null
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t_girl(# union all
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t_girl(# select a.id,q.path||‘/‘||b.name as "path" from country_relation as a inner join tmp_country as q on (q.id = a.parentid)
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t_girl(# inner join country as b on (a.id = b.id)
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t_girl(# )
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t_girl-# select a.path from tmp_country as a;
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path
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-----------------------------------------------
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/Earth
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/Earth/North America
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/Earth/South America
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/Earth/Europe
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/Earth/Asia
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/Earth/Africa
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/Earth/Australia
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/Earth/North America/Canada
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/Earth/North America/Central America
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/Earth/North America/Island Nations
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/Earth/North America/United States
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/Earth/North America/United States/Alabama
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/Earth/North America/United States/Alaska
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/Earth/North America/United States/Arizona
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/Earth/North America/United States/Arkansas
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/Earth/North America/United States/California
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(16 rows)
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Time: 3.260 ms
还可以用tablefunc扩展带来的CONNECT BY函数把这棵树遍历出来。
由于昨天设计的两张表通过ID来关联，这个扩展自带的函数要把名字展现出来比较麻烦，索性这里我就用了一张临时表保存我想要的结果。
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t_girl=# CREATE TEMPORARY TABLE tmp_country_relation as SELECT b.id,a.name,b.parentid,‘‘::text as parentname FROM country AS a,country_relation AS b WHERE a.id = b.id;
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SELECT 16
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Time: 11.773 ms
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t_girl=#
这里更新了对应的ID为NAME。
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t_girl=# update tmp_country_relation set parentname = a.name from country as a where parentid = a.id;
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UPDATE 15
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Time: 1.829 ms
我用TABLEFUNC扩展带来的CONNECT BY 实现这棵树的遍历。
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t_girl=# select path from connectby(‘tmp_country_relation as a‘,‘a.name‘,‘a.parentname‘,‘Earth‘,0,‘/‘) as g(id text,parentid text,level int,path text) order by level;
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path
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----------------------------------------------
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Earth
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Earth/Australia
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Earth/North America
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Earth/Africa
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Earth/South America
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Earth/Europe
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Earth/Asia
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Earth/North America/Island Nations
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Earth/North America/Canada
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Earth/North America/Central America
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Earth/North America/United States
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Earth/North America/United States/California
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Earth/North America/United States/Arkansas
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Earth/North America/United States/Alabama
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Earth/North America/United States/Alaska
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Earth/North America/United States/Arizona
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(16 rows)
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Time: 5.974 ms
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t_girl=#
【原创】PostgreSQL 对简单树的遍历
原文：http://blog.chinaunix.net/uid-259788-id-4599148.html
