昨天我用MysqL來實現了ORACLE的遞歸語句CONNECT BY， 看起來稍復雜些。今天來看看POSTGREsql如何實現ORACLE的CONNECT BY。

還是用昨天同樣的表以及數據。POSTGREsql自詡最像ORACLE的數據庫，所以大部分語句也就都可以簡單而且變相的實現了。

在這點上可以用他自己帶的WITH遞歸功能，還可以用第三方擴展帶來的類似connect by 函數。

先來看第一點，用遞歸的WITH來展現這棵樹的路徑。

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 null

t_girl(# union all

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)

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

還可以用tablefunc擴展帶來的CONNECT BY函數把這棵樹遍歷出來。

由于昨天設計的兩張表通過ID來關聯，這個擴展自帶的函數要把名字展現出來比較麻煩，索性這里我就用了一張臨時表保存我想要的結果。

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 16

Time: 11.773 ms

t_girl=#

這里更新了對應的ID為NAME。

t_girl=# update tmp_country_relation set parentname = a.name from country as a where parentid = a.id;

UPDATE 15

Time: 1.829 ms

我用TABLEFUNC擴展帶來的CONNECT BY 實現這棵樹的遍歷。

t_girl=# select path from connectby('tmp_country_relation as a','a.name','a.parentname','Earth','/') 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 ms

t_girl=#

總結

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