层次化查询，顾名思义就是把查询结果有层次的呈现出来。层次化查询结果类似于树状结构，最顶端的是“根节点”，下面是“父节点”，没有子节点的是“叶节点”。
为了让一个或多个表具有层次关系，必须使用相关的字段将表关联起来。比如HR用户下的employees表中的employee_id和manager_id列。
基本语法为：
[START WITH conditiona1] CONNECT BY condition2
START WITH conditiona1:用来指定根节点，condition1中可以加子查询。
CONNECT BY condition2：condition2中必须包含PRIOR关键字，用来指定列是来自父行。PRIOR可以位于比较操作符的任何一方。condition不能包含子查询。
如：
SQL> select employee_id,manager_id,first_name,last_name,hire_date
2 from employees
3 start with manager_id is null
4 connect by prior employee_id=manager_id;
EMPLOYEE_ID MANAGER_ID FIRST_NAME LAST_NAME HIRE_DATE
----------- ---------- -------------------- ------------------------- -------------------
100 Steven King 2003-06-17 00:00:00
101 100 Neena Kochhar 2005-09-21 00:00:00
108 101 Nancy Greenberg 2002-08-17 00:00:00
109 108 Daniel Faviet 2002-08-16 00:00:00
110 108 John Chen 2005-09-28 00:00:00
111 108 Ismael Sciarra 2005-09-30 00:00:00
112 108 Jose Manuel Urman 2006-03-07 00:00:00
113 108 Luis Popp 2007-12-07 00:00:00
200 101 Jennifer Whalen 2003-09-17 00:00:00
203 101 Susan Mavris 2002-06-07 00:00:00
为了知道当前查询树状结构所处的层次，可以使用level伪列。加入level伪列后显示如下。
SQL> select level,lpad(' ',2*(level-1))||last_name "EmpName",hire_date,salary
2 from employees
3 start with manager_id is null
4 connect by manager_id=prior employee_id;
LEVEL EmpName HIRE_DATE SALARY
---------- --------------- ------------------- ----------
1 King 2003-06-17 00:00:00 24000
2 Kochhar 2005-09-21 00:00:00 17000
3 Greenberg 2002-08-17 00:00:00 12008
4 Faviet 2002-08-16 00:00:00 9000
4 Chen 2005-09-28 00:00:00 8200
4 Sciarra 2005-09-30 00:00:00 7700
4 Urman 2006-03-07 00:00:00 7800
4 Popp 2007-12-07 00:00:00 6900
3 Whalen 2003-09-17 00:00:00 4400
3 Mavris 2002-06-07 00:00:00 6500
3 Baer 2002-06-07 00:00:00 10000
3 Higgins 2002-06-07 00:00:00 1200