怎么用PostgreSQL对树进行遍历

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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。

t_girl=# with recursive tmp_country(id,path) as t_girl-# (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 nullt_girl(# union allt_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)t_girl(# inner join country as b on (a.id = b.id)t_girl(# )t_girl-# select a.path from tmp_country as a;                     path                      ----------------------------------------------- /Earth /Earth/North America /Earth/South America /Earth/Europe /Earth/Asia /Earth/Africa /Earth/Australia /Earth/North America/Canada /Earth/North America/Central America /Earth/North America/Island Nations /Earth/North America/United States /Earth/North America/United States/Alabama /Earth/North America/United States/Alaska /Earth/North America/United States/Arizona /Earth/North America/United States/Arkansas /Earth/North America/United States/California(16 rows)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。

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;      SELECT 16Time: 11.773 mst_girl=#

这里更新了对应的ID为NAME。

t_girl=# update tmp_country_relation set parentname = a.name from country as a where parentid = a.id;UPDATE 15Time: 1.829 ms

我用TABLEFUNC扩展带来的CONNECT BY 实现这棵树的遍历。

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;                        path                     ---------------------------------------------- Earth Earth/Australia Earth/North America Earth/Africa Earth/South America Earth/Europe Earth/Asia Earth/North America/Island Nations Earth/North America/Canada Earth/North America/Central America Earth/North America/United States Earth/North America/United States/California Earth/North America/United States/Arkansas Earth/North America/United States/Alabama Earth/North America/United States/Alaska Earth/North America/United States/Arizona(16 rows)Time: 5.974 mst_girl=#

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