為什么80%的碼農都做不了架構師？>>> ??

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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,用于表示樹的層次

?????? 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系

???

??? --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(通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記錄的相關信息)

?????? 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等于父節點的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應當等于前一條記錄的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應當等于前一條記錄的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置于等號右邊,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'??? --通過where子句來過濾SCOTT用戶，但SCOTT的下屬ADAMS并沒有過濾掉

??? ? 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.

????

??? --通過將過濾條件由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.

???

??? 更多參考：

?

Oracle 數據庫實例啟動關閉過程

?

Oracle 10g SGA 的自動化管理

?

使用OEM,SQL*Plus,iSQL*Plus 管理Oracle實例

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Oracle實例和Oracle數據庫(Oracle體系結構)

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SQL 基礎-->常用函數

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SQL基礎-->過濾和排序

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SQL 基礎-->SELECT 查詢

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原文鏈接： http://blog.csdn.net/robinson_0612/article/details/5616877

轉載于:https://my.oschina.net/dtec/blog/47452

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