Oracle “CONNECT BY” 使用
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
现在我要根据“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 /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 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：
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
