查詢員工編號為7369的領導:

1 SELECT LEVEL,E.* FROM EMP E CONNECT BY PRIOR E.MGR = E.EMPNO  START WITH E.EMPNO = 7876
2 ORDER BY LEVEL DESC

"start with" -- this identifies all LEVEL=1 nodes in the tree

"connect by" -- describes how to walk from the parent nodes above to their children and 
their childrens children.

Easiest to use an example on emp. If we start with "where mgr is NULL", we generate the 
set of employees that have no mgr (they are the top of the tree). If we

CONNECT BY PRIOR EMPNO = /* current */ MGR

that will take all of the PRIOR records (the start with at first) and find all records 
such that the MGR column equals their EMPNO (find all the records of people managed by 
the people we started with).

使用WITH語句優化查詢結果:優化等級

 1 WITH A AS
 2  (SELECT MAX(LEVEL) + 1 LVL
 3     FROM EMP E
 4   CONNECT BY PRIOR E.MGR = E.EMPNO
 5    START WITH E.EMPNO = 7876
 6    ORDER BY LEVEL DESC)
 7 SELECT A.LVL 最高等級加1,
 8        LEVEL 當前等級,
 9        A.LVL - LEVEL 優化後等級,
10        E.* 　FROM A,
11        EMP E CONNECT BY
 
 PRIOR E.MGR = E.EMPNO START WITH E.EMPNO = 7876 ORDER BY LEVEL DESC

查詢員工編號為7839的所有下屬(7839為king):

1 SELECT LEVEL 等級, E.*
2   FROM EMP E
3 CONNECT BY PRIOR E.EMPNO = E.MGR
4  START WITH E.EMPNO = 7839

--構造整個的層次結構

1 select lpad(' ',level*2,' ')||ename ename, empno, mgr
2     from emp
3     START WITH MGR IS NULL
4     CONNECT BY PRIOR EMPNO = MGR

So, KING is the start with set then JONES BLAKE and CLARK fall under him. Each of them 
becomes the PRIOR record in turn and their trees are expanded.

使用Connect By 結合 level構造虛擬行:

1 SELECT LEVEL FROM DUAL CONNECT BY LEVEL < 5

使用rownum實現類似的功能:

1 SELECT LEVEL FROM DUAL CONNECT BY LEVEL < 5

---------------------待續-----------------------

使用UNION ALL構造兩層節點的樹:

檢視如下所示:

 1 CREATE OR REPLACE VIEW TREE_VIEW AS
 2 SELECT
 3  '1' AS rootnodeid,
 4  'xxxx有限責任公司' AS treename,
 5  '-1'  AS parent_id
 6 FROM dual
 7 UNION
 8 SELECT
 9   to_char(d.deptno),
10   d.dname || '_' ||d.loc,
11   '1' AS parent_id
12  FROM dept d;

查詢語句:

1 SELECT T.*, LEVEL
2   FROM TREE_VIEW T
3  START WITH T.PARENT_ID = '-1'
4 CONNECT BY PRIOR T.ROOTNODEID = T.PARENT_ID

-----以下為更新內容:

1、先檢視總共有幾個等級：

1 SELECT COUNT(LEVEL)
2   FROM EMP E
3 CONNECT BY PRIOR E.EMPNO = E.MGR
4  START WITH E.MGR IS NULL;

2、檢視每個等級的人數。主要是通過LEVEL進行GROUP BY

1 SELECT COUNT(LEVEL)
2   FROM EMP E
3 CONNECT BY PRIOR E.EMPNO = E.MGR
4  START WITH E.MGR IS NULL
5  GROUP BY LEVEL;

3、Oracle 10g提供了一個簡單的connect_by_isleaf=1,

0 表示非葉子節點

1 SELECT LEVEL AS 等級, CONNECT_BY_ISLEAF AS 是否是葉子節點, E.*
2   FROM EMP E
3 CONNECT BY PRIOR E.EMPNO = E.MGR
4  START WITH E.MGR IS NULL

4、SYS_CONNECT_BY_PATH

Oracle 9i提供了sys_connect_by_path(column,char),其中column 是字元型或能自動轉

換成字元型的列名。它的主要目的就是將父節點到當前節點的”path”按照指定的模式展現出現。這個函式只能使用在層次查詢中。

1 SELECT LEVEL AS 等級,
2        CONNECT_BY_ISLEAF AS 是否是葉子節點,
3        LPAD(' ', LEVEL * 2 - 1) || SYS_CONNECT_BY_PATH(ENAME, '=>')
4   FROM EMP E
5 CONNECT BY PRIOR E.EMPNO = E.MGR
6  START WITH E.MGR IS NULL;

5、修剪樹枝和節點:

    過濾掉編號是7566的資料(修剪節點)，他指的是把這個節點給裁掉，但是並沒有破壞樹結構，它的子節點還是可以正常的顯示。

1 SELECT LEVEL AS 等級,
2        CONNECT_BY_ISLEAF AS 是否是葉子節點,
3        LPAD(' ', LEVEL * 2 - 1) || SYS_CONNECT_BY_PATH(ENAME, '=>'),
4        E.*
5   FROM EMP E
6 WHERE e.empno != 7566
7 CONNECT BY PRIOR E.EMPNO = E.MGR
8  START WITH E.MGR IS NULL;

裁掉編號是7698的節點和它的子節點:

1 SELECT LEVEL AS 等級,
2        CONNECT_BY_ISLEAF AS 是否是葉子節點,
3        LPAD(' ', LEVEL * 2 - 1) || SYS_CONNECT_BY_PATH(ENAME, '=>'),
4        E.*
5   FROM EMP E
6 CONNECT BY PRIOR E.EMPNO = E.MGR
7        AND E.EMPNO != 7698
8  START WITH E.MGR IS NULL;

6、CONNECT_BY_ROOT的使用，oracle10g新增connect_by_root,用在列名之前表示此行的根節點的相同列名的值。

1 SELECT LEVEL AS 等級,
2        CONNECT_BY_ISLEAF AS 是否是葉子節點,
3        CONNECT_BY_ROOT ENAME,
4        LPAD(' ', LEVEL * 2 - 1) || SYS_CONNECT_BY_PATH(ENAME, '=>'),
5        E.*
6   FROM EMP E
7 CONNECT BY PRIOR E.EMPNO = E.MGR
8  START WITH E.MGR IS NULL;

對於層次查詢如果用order by排序，比如order by last_name則是先做完層次獲得level,然後按last_name 排序，這樣破壞了層次，比如特別關注某行的深度，按level 排序，也是會破壞層次的。在oracle10g中，增加了siblings 關鍵字的排序。

語法：order siblings by <expre>

它會保護層次，並且在每個等級中按expre排序。

1 SELECT LEVEL AS 等級,
2        CONNECT_BY_ISLEAF AS 是否是葉子節點,
3        LPAD(' ', LEVEL * 2 - 1) || SYS_CONNECT_BY_PATH(ENAME, '=>'),
4        E.*
5   FROM EMP E
6 CONNECT BY PRIOR E.EMPNO = E.MGR 
7  START WITH E.MGR IS NULL
8  ORDER SIBLINGS BY  E.ENAME;

connect_by_iscycle(存在迴圈，將返回1，否則返回0)

The CONNECT_BY_ISCYCLE pseudocolumn returns 1 if the current row has a child which is also its ancestor. Otherwise it returns 0. 
You can specify CONNECT_BY_ISCYCLE only if you have specified the NOCYCLE parameter of the CONNECT BY clause. NOCYCLE enables Oracle to return the results of a query that would otherwise fail because of a CONNECT BY loop in the data.