我们可以通过START WITH . . . CONNECT BY . . .子句来实现SQL的 层次查询，而Oracle 10g 为其添加许多了新的伪列。十多年以来，Oracle SQL 具有依照层次关系进行查询的功能。例如，你可以指定一个起始条件，然后根据一个或多个连接条件来确定孩子行的内容。举例来说，现在假设我有一个表，里面记录了世界上的某些地区，其表结构如下：
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');
那么我们可以使用START WITH . . . CONNECT BY . . .从句将父级地区与孩子地区连接起来，并将其层次等级显示出来。
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
