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--SQL基礎-->層次化查詢(START BY ... CONNECT BY PRIOR)

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層次化查詢,即樹型結構查詢,是SQL中經常用到的功能之一,通常由根節點,父節點,子節點,葉節點組成,其語法如下:

SELECT [LEVEL] ,column,expression,...

FROM table_name

[WHERE where_clause]

[[START WITH start_condition] [CONNECT BY PRIOR prior_condition]];

LEVEL:為偽列,用於表示樹的層次

start_condition：層次化查詢的起始條件

prior_condition：定義父節點和子節點之間的關係

--使用start with ...connect by prior 從根節點開始遍歷

SQL> select empno,mgr,ename,job from emp

2 start with empno = 7839

3 connect by prior empno = mgr;

EMPNO MGR ENAME JOB

---------- ---------- ---------- ---------

7839 KING PRESIDENT

7566 7839 JONES MANAGER

7788 7566 SCOTT ANALYST

7876 7788 ADAMS CLERK

7902 7566 FORD ANALYST

7369 7902 SMITH CLERK

7698 7839 BLAKE MANAGER

7499 7698 ALLEN SALESMAN

7521 7698 WARD SALESMAN

7654 7698 MARTIN SALESMAN

7844 7698 TURNER SALESMAN

EMPNO MGR ENAME JOB

---------- ---------- ---------- ---------

7900 7698 JAMES CLERK

7782 7839 CLARK MANAGER

7934 7782 MILLER CLERK

14 rows selected.

樹型結構遍歷過程(透過上面的查詢來描述)

1).從根節點開始(即where_clause中的條件,如果為非根節點則分根節點作為根節點開始遍歷,如上例empno = 7839)

2).遍歷根節點(得到empno = 7839記錄的相關資訊)

3).判斷該節點是否存在由子節點，如果則訪問最左側未被訪問的子節點,轉到),否則下一步

如上例中prior_condition為empno = mgr,即子節點的mgr等於父節點的empno,在此時mgr為7839的記錄

4).當節點為葉節點，則訪問完畢,否則,轉到)

5).返回到該節點的父節點,轉到)

--偽列level的使用

--注意connect by prior empno = mgr 的理解

--prior表示前一條記錄,即下一條返回記錄的mgr應當等於前一條記錄的empno

SQL> select level,empno,mgr,ename,job from emp

2 start with ename = 'KING'

3 connect by prior empno = mgr

4 order by level;

LEVEL EMPNO MGR ENAME JOB

---------- ---------- ---------- ---------- ---------

1 7839 KING PRESIDENT

2 7566 7839 JONES MANAGER

2 7698 7839 BLAKE MANAGER

2 7782 7839 CLARK MANAGER

3 7902 7566 FORD ANALYST

3 7521 7698 WARD SALESMAN

3 7900 7698 JAMES CLERK

3 7934 7782 MILLER CLERK

3 7499 7698 ALLEN SALESMAN

3 7788 7566 SCOTT ANALYST

3 7654 7698 MARTIN SALESMAN

LEVEL EMPNO MGR ENAME JOB

---------- ---------- ---------- ---------- ---------

3 7844 7698 TURNER SALESMAN

4 7876 7788 ADAMS CLERK

4 7369 7902 SMITH CLERK

--獲得層次數

SQL> select count(distinct level) "Level" from emp

2 start with ename = 'KING'

3 connect by prior empno = mgr;

Level

----------

4

--格式化層次查詢結果(使用左填充* level - 1個空格)

SQL> col Ename for a30

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename",

3 job

4 from emp

5 start with ename = 'KING'

6 connect by prior empno = mgr;

LEVEL Ename JOB

---------- ------------------------------ ---------

1 KING PRESIDENT

2 JONES MANAGER

3 SCOTT ANALYST

4 ADAMS CLERK

3 FORD ANALYST

4 SMITH CLERK

2 BLAKE MANAGER

3 ALLEN SALESMAN

3 WARD SALESMAN

3 MARTIN SALESMAN

3 TURNER SALESMAN

LEVEL Ename JOB

---------- ------------------------------ ---------

3 JAMES CLERK

2 CLARK MANAGER

3 MILLER CLERK

14 rows selected.

--從非根節點開始遍歷(只需修改start with 中的條件即可)

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename",

3 job

4 from emp

5 start with ename = 'SCOTT'

6 connect by prior empno = mgr;

LEVEL Ename JOB

---------- ------------------------------ ---------

1 SCOTT ANALYST

2 ADAMS CLERK

--從下向上遍歷(交換connect by prior中的條件即可,使用mgr = empno)

--注意connect by prior mgr = empno 的理解
--prior表示前一條記錄,即下一條返回記錄的empno應當等於前一條記錄的mgr

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename",

3 job

4 from emp

5 start with ename = 'SCOTT'

6 connect by prior mgr = empno;

LEVEL Ename JOB

---------- ------------------------------ ---------

1 SCOTT ANALYST

2 JONES MANAGER

3 KING PRESIDENT

--從下向上遍歷(也可以將prior置於等號右邊,得到相同的結果)

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename",

3 job

4 from emp

5 start with ename = 'SCOTT'

6 connect by empno = prior mgr;

LEVEL Ename JOB

---------- ------------------------------ ---------

1 SCOTT ANALYST

2 JONES MANAGER

3 KING PRESIDENT

--從層次查詢中刪除節點和分支

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename"

3 ,job

4 from emp

5 where ename != 'SCOTT' --透過where子句來過濾SCOTT使用者，但SCOTT的下屬ADAMS並沒有過濾掉

6 start with empno = 7839

7 connect by prior empno = mgr;

LEVEL Ename JOB

---------- -------------------- ---------

1 KING PRESIDENT

2 JONES MANAGER

4 ADAMS CLERK

3 FORD ANALYST

4 SMITH CLERK

2 BLAKE MANAGER

3 ALLEN SALESMAN

3 WARD SALESMAN

3 MARTIN SALESMAN

3 TURNER SALESMAN

3 JAMES CLERK

LEVEL Ename JOB

---------- -------------------- ---------

2 CLARK MANAGER

3 MILLER CLERK

13 rows selected.

--透過將過濾條件由where 子句的內容移動到connect by prior 子句中過濾掉SCOTT及其下屬

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename"

3 ,job

4 from emp

5 start with empno = 7839

6 connect by prior empno = mgr and ename != 'SCOTT';

LEVEL Ename JOB

---------- -------------------- ---------

1 KING PRESIDENT

2 JONES MANAGER

3 FORD ANALYST

4 SMITH CLERK

2 BLAKE MANAGER

3 ALLEN SALESMAN

3 WARD SALESMAN

3 MARTIN SALESMAN

3 TURNER SALESMAN

3 JAMES CLERK

2 CLARK MANAGER

LEVEL Ename JOB

---------- -------------------- ---------

3 MILLER CLERK

12 rows selected.

--在層次化查詢中增加過濾條件或使用子查詢

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename"

3 ,job

4 from emp

5 where sal > 2500

6 start with empno = 7839

7 connect by prior empno = mgr

8 ;

LEVEL Ename JOB

---------- -------------------- ---------

1 KING PRESIDENT

2 JONES MANAGER

3 SCOTT ANALYST

3 FORD ANALYST

2 BLAKE MANAGER

SQL> select level,

2 lpad(' ',2 * level - 1) || ename as "Ename"

3 ,job

4 from emp

5 where sal > (select avg(sal) from emp)

6 start with empno = 7839

7 connect by prior empno = mgr ;

LEVEL Ename JOB

---------- -------------------- ---------

1 KING PRESIDENT

2 JONES MANAGER

3 SCOTT ANALYST

3 FORD ANALYST

2 BLAKE MANAGER

2 CLARK MANAGER

6 rows selected.

轉載：http://blog.csdn.net/leshami/article/details/5616877

【--SQL基礎-->層次化查詢(START BY ... CONNECT BY PRIOR)】

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