oracle数据库树状或者层次结果集的查询

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：

[ START WITH condition ]
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

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

初始化一些数据

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (0, '总经办', null);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (1, '开发部', 0);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (2, '测试部', 0);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (3, 'Sever开发部', 1);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (4, 'Client开发部', 1);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (5, 'TA测试部', 2);
1 row inserted

SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (6, '项目测试部', 2);
1 row inserted

SQL> commit;
Commit complete
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 rows selected要根据“CONNECT BY”来实现树状查询结果

SQL> SELECT RPAD( ’ ‘, 2*(LEVEL-1), ‘-’ ) || DEPNAME “DEPNAME”,
CONNECT_BY_ROOT DEPNAME “ROOT”,
CONNECT_BY_ISLEAF “ISLEAF”,
LEVEL ,
SYS_CONNECT_BY_PATH(DEPNAME, ‘/’) “PATH”
FROM DEP
START WITH UPPERDEPID IS NULL
CONNECT BY PRIOR DEPID = UPPERDEPID;



DEPNAME                ROOT             ISLEAF    LEVEL   PATH
------------------------------ ------------------- ---------- -------
General Deparment     General Deparment  0          1
-Development          General Deparment  0          2
---Server Development General Deparment  1          3
---Client Development General Deparment  1          3
-QA                   General Deparment  0          2
---TA                 General Deparment  1          3
---Porject QA         General Deparment  1          3

7 rows selected
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

SQL> SELECT ROWNUM FROM DUAL CONNECT BY ROWNUM <= 10;



    ROWNUM
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10

10 rows selected
[例3]

通过CONNECT BY用于十六进度转换为十进制

CREATE OR REPLACE FUNCTION f_hex_to_dec(p_str IN VARCHAR2) 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
SELECT SUM(DATA) INTO v_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));
RETURN v_return;
EXCEPTION
WHEN OTHERS THEN
RETURN NULL;
END;
“`

说明：

CONNECT BY rownum <= length(p_str))对输入的字符串进行逐个遍历

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通过CASE语句，来解析十六进制中的A-F对应的10进制值

oracle数据库树状或者层次结果集的查询

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