Oracle “CONNECT BY” 使用

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Oracle “CONNECT BY”是层次查询子句，一般用于树状或者层次结果集的查询。其语法是：

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[ START WITHcondition ]
CONNECT BY [ NOCYCLE ] condition

The start with .. connect by clause can be used to select data that has a hierarchical relationship (usually some sort of parent->child (boss->employee or thing->parts).

说明：
1. START WITH：告诉系统以哪个节点作为根结点开始查找并构造结果集，该节点即为返回记录中的最高节点。
2. 当分层查询中存在上下层互为父子节点的情况时，会返回ORA-01436错误。此时，需要在connect by后面加上NOCYCLE关键字。同时，可用connect_by_iscycle伪列定位出存在互为父子循环的具体节点。 connect_by_iscycle必须要跟关键字NOCYCLE结合起来使用

接下来，用一些示例来说明“CONNECT BY”的用法。

[例1]

创建一个部门表，这个表有三个字段，分别对应部门ID，部门名称，以及上级部门ID

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-- Create table
create table DEP
(
  DEPID      number(10)not null,
  DEPNAME    varchar2(256),
  UPPERDEPID number(10)
)
;

初始化一些数据

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SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (0, '总经办', null);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (1, '开发部', 0);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (2, '测试部', 0);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (3, 'Sever开发部', 1);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (4, 'Client开发部', 1);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (5, 'TA测试部', 2);
1 row inserted
SQL> INSERTINTO DEP(DEPID, DEPNAME, UPPERDEPID)VALUES (6, '项目测试部', 2);
1 row inserted
SQL> commit;
Commit complete

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SQL> SELECT* FROM DEP;
      DEPID DEPNAME                                                                           UPPERDEPID
----------- -------------------------------------------------------------------------------- -----------
          0 General Deparment                                                               
          1 Development                                                                                0
          2 QA                                                                                         0
          3 Server Development                                                                         1
          4 Client Development                                                                         1
          5 TA                                                                                         2
          6 Porject QA                                                                                 2
7 rowsselected

现在我要根据“CONNECT BY”来实现树状查询结果

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SQL> SELECTRPAD( ' ', 2*(LEVEL-1),'-' ) || DEPNAME "DEPNAME",
CONNECT_BY_ROOT DEPNAME"ROOT",
CONNECT_BY_ISLEAF "ISLEAF",
LEVEL ,
SYS_CONNECT_BY_PATH(DEPNAME,'/') "PATH"
FROM DEP
START WITHUPPERDEPID IS NULL
CONNECT BY PRIOR DEPID = UPPERDEPID;
DEPNAME                        ROOT                    ISLEAF     LEVEL PATH
------------------------------ ------------------- ---------- ---------- --------------------------------------------------------------------------------
General Deparment              General Deparment            0          1 /General Deparment
-Development                  General Deparment            0          2 /General Deparment/Development
---Server Development         General Deparment            1          3 /General Deparment/Development/Server Development
---Client Development         General Deparment            1          3 /General Deparment/Development/Client Development
-QA                           General Deparment            0          2 /General Deparment/QA
---TA                         General Deparment            1          3 /General Deparment/QA/TA
---Porject QA                 General Deparment            1          3 /General Deparment/QA/Porject QA
                                                  
7 rowsselected

说明：
1. CONNECT_BY_ROOT 返回当前节点的最顶端节点
2. CONNECT_BY_ISLEAF 判断是否为叶子节点，如果这个节点下面有子节点，则不为叶子节点
3. LEVEL 伪列表示节点深度
4. SYS_CONNECT_BY_PATH函数显示详细路径，并用“/”分隔

[例2]

通过CONNECT BY生成序列

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SQL> SELECTROWNUM FROM DUAL CONNECT BYROWNUM <= 10;
    ROWNUM
----------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
10 rowsselected

[例3]

通过CONNECT BY用于十六进度转换为十进制

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CREATE OR REPLACE FUNCTIONf_hex_to_dec(p_str INVARCHAR2) RETURN VARCHAR2 IS
    ----------------------------------------------------------------------------------------------------------------------
    -- 对象名称: f_hex_to_dec
    -- 对象描述: 十六进制转换十进制
    -- 输入参数: p_str 十六进制字符串
    -- 返回结果: 十进制字符串
    -- 测试用例: SELECT f_hex_to_dec('78A') FROM dual;
    ----------------------------------------------------------------------------------------------------------------------
    v_return  VARCHAR2(4000);
  BEGIN
    SELECTSUM(DATA) INTOv_return
      FROM(SELECT (CASE upper(substr(p_str, rownum, 1))
                    WHEN 'A' THEN '10'
                    WHEN 'B' THEN '11'
                    WHEN 'C' THEN '12'
                    WHEN 'D' THEN '13'
                    WHEN 'E' THEN '14'
                    WHEN 'F' THEN '15'
                    ELSE substr(p_str, rownum, 1)
                  END) * power(16, length(p_str) - rownum) DATA
              FROM dual
            CONNECT BY rownum <= length(p_str));
    RETURNv_return;
  EXCEPTION
    WHENOTHERS THEN
      RETURNNULL;
  END;

说明：
1. CONNECT BY rownum <= length(p_str))对输入的字符串进行逐个遍历
2. 通过CASE语句，来解析十六进制中的A-F对应的10进制值

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