Oracle “CONNECT BY” (层级递归查询)

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：

1 2	`[ 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