我們可以通過START WITH . . . CONNECT BY . . .子句來實(shí)現(xiàn)SQL的 層次查詢，而Oracle 10g 為其添加許多了新的偽列。十多年以來，Oracle SQL 具有依照層次關(guān)系進(jìn)行查詢的功能。例如，你可以指定一個(gè)起始條件，然后根據(jù)一個(gè)或多個(gè)連接條件來確定孩子行的內(nèi)容。 舉例來說，現(xiàn)在假設(shè)我有一個(gè)表，里面記錄了世界上的某些地區(qū)，其表結(jié)構(gòu)如下：
create table hier
(
parent varchar2(30),
child varchar2(30)
);
insert into hier values(null,'Asia');
insert into hier values(null,'Australia');
insert into hier values(null,'Europe');
insert into hier values(null,'North America');
insert into hier values('Asia','China');
insert into hier values('Asia','Japan');
insert into hier values('Australia','New South Wales');
insert into hier values('New South Wales','Sydney');
insert into hier values('California','Redwood Shores');
insert into hier values('Canada','Ontario');
insert into hier values('China','Beijing');
insert into hier values('England','London');
insert into hier values('Europe','United Kingdom');
insert into hier values('Japan','Osaka');
insert into hier values('Japan','Tokyo');
insert into hier values('North America','Canada');
insert into hier values('North America','USA');
insert into hier values('Ontario','Ottawa');
insert into hier values('Ontario','Toronto');
insert into hier values('USA','California');
insert into hier values('United Kingdom','England');
那么我們可以使用START WITH . . . CONNECT BY . . .從句將父級地區(qū)與孩子地區(qū)連接起來，并將其層次等級顯示出來。
column child format a40
select level,lpad(' ',level*3)child child
from hier
start with parent is null
connect by PRior child = parent;
LEVEL CHILD
---------- --------------------------
1 Asia
2 China
3 Beijing
2 Japan
3 Osaka
3 Tokyo
1 Australia
2 New South Wales
3 Sydney
1 Europe
2 United Kingdom
3 England
4 London
1 North America
2 Canada
3 Ontario
4 Ottawa
4 Toronto
2 USA
3 California
4 Redwood Shores
自從Since Oracle 9i 開始，就可以通過 SYS_CONNECT_BY_PATH 函數(shù)實(shí)現(xiàn)將從父節(jié)點(diǎn)到當(dāng)前行內(nèi)容以“path”或者層次元素列表的形式顯示出來。 如下例所示：
column path format a50
select level,sys_connect_by_path(child,'/') path
from hier
start with parent is null
connect by prior child = parent;
LEVEL PATH
-------- --------------------------------------------
1 /Asia
2 /Asia/China
3 /Asia/China/Beijing
2 /Asia/Japan
3 /Asia/Japan/Osaka
3 /Asia/Japan/Tokyo
1 /Australia
2 /Australia/New South Wales
3 /Australia/New South Wales/Sydney
1 /Europe
2 /Europe/United Kingdom
3 /Europe/United Kingdom/England
4 /Europe/United Kingdom/England/London
1 /North America
2 /North America/Canada
3 /North America/Canada/Ontario
4 /North America/Canada/Ontario/Ottawa
4 /North America/Canada/Ontario/Toronto
2 /North America/USA
3 /North America/USA/California
4 /North America/USA/California/Redwood Shores
在 Oracle 10g 中，還有其他更多關(guān)于層次查詢的新特性 。
例如，有的時(shí)候用戶更關(guān)心的是每個(gè)層次分支中等級最低的內(nèi)容。那么你就可以利用偽列函數(shù)CONNECT_BY_ISLEAF來判定當(dāng)前行是不是葉子。假如是葉子就會(huì)在偽列中顯示“1”，假如不是葉子而是一個(gè)分支（例如當(dāng)前內(nèi)容是其他行的父親）就顯示“0”。下給出了一個(gè)關(guān)于這個(gè)函數(shù)使用的例子：
select connect_by_isleaf,sys_connect_by_path(child,'/') path
from hier
start with parent is null
connect by prior child = parent;
CONNECT_BY_ISLEAF PATH
---------------------------------- ------------
0 /Asia
0 /Asia/China
1 /Asia/China/Beijing
0 /Asia/Japan
1 /Asia/Japan/Osaka
1 /Asia/Japan/Tokyo
0 /Australia
0 /Australia/New South Wales
1 /Australia/New South Wales/Sydney
0 /Europe
0 /Europe/United Kingdom
0 /Europe/United Kingdom/England
1 /Europe/United Kingdom/England/London
0 /North America
0 /North America/Canada
0 /North America/Canada/Ontario
1 /North America/Canada/Ontario/Ottawa
1 /North America/Canada/Ontario/Toronto
0 /North America/USA
0 /North America/USA/California
1 /North America/USA/California/Redwood Shores
在Oracle 10g 中還有一個(gè)新操作——CONNECT_BY_ROOT。 它用在列名之前用于返回當(dāng)前層的根節(jié)點(diǎn)。如下面的例子，我可以顯示出層次結(jié)構(gòu)表中當(dāng)前行數(shù)據(jù)所對應(yīng)的最高等級節(jié)點(diǎn)的內(nèi)容。
select connect_by_root child,sys_connect_by_path(child,'/') path
from hier
start with parent is null
connect by prior child = parent;
CONNECT_BY_ROOT PATH
------------------------------ --------
Asia /Asia
Asia /Asia/China
Asia /Asia/China/Beijing
Asia /Asia/Japan
Asia /Asia/Japan/Osaka
Asia /Asia/Japan/Tokyo
Australia /Australia
Australia /Australia/New South Wales
Australia /Australia/New South Wales/Sydney
Europe /Europe
Europe /Europe/United Kingdom
Europe /Europe/United Kingdom/England
Europe /Europe/United Kingdom/England/London
North America /North America
North America /North America/Canada
North America /North America/Canada/Ontario
North America /North America/Canada/Ontario/Ottawa
North America /North America/Canada/Ontario/Toronto
North America /North America/USA
North America /North America/USA/California
North America /North America/USA/California/Redwood Shores
在Oracle 10g 之前的版本中，假如在你的樹中出現(xiàn)了環(huán)狀循環(huán)（如一個(gè)孩子節(jié)點(diǎn)引用一個(gè)父親節(jié)點(diǎn)），Oracle 就會(huì)報(bào)出一個(gè)錯(cuò)誤提示：“ ORA-01436: CONNECT BY loop in user data”。假如不刪掉對父親的引用就無法執(zhí)行查詢操作。而在 Oracle 10g 中，只要指定“NOCYCLE”就可以進(jìn)行任意的查詢操作。與這個(gè)要害字相關(guān)的還有一個(gè)偽列——CONNECT_BY_ISCYCLE， 假如在當(dāng)前行中引用了某個(gè)父親節(jié)點(diǎn)的內(nèi)容并在樹中出現(xiàn)了循環(huán)，那么該行的偽列中就會(huì)顯示“1”，否則就顯示“0”。如下例所示：
create table hier2
(
parent number,
child number
);
insert into hier2 values(null,1);
insert into hier2 values(1,2);
insert into hier2 values(2,3);
insert into hier2 values(3,1);
select connect_by_iscycle,sys_connect_by_path(child,'/') path
from hier2
start with parent is null
connect by nocycle prior child = parent;
CONNECT_BY_ISCYCLE PATH
------------------ -------
0 /1
0 /1/2
1 /1/2/3
