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 WITHcondition ]CONNECT BY [ NOCYCLE ] condition The start with .. connect by clause can be used to select data that has a hierarchical relationship (
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 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
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)notnull, 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 rowsselected
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 LEVELPATH
------------------------------ ------------------- ---------- ---------- --------------------------------------------------------------------------------
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
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> SELEC TROWNUM FROM DUAL CONNECT BY ROWNUM
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_strINVARCHAR2)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 (CASEupper(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 说明：
1. CONNECT BY rownum
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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