在ORACLE数据库查询中,我们经常会遇到对树型结构表的查询,这是个麻烦的问题。下面给大家介绍一种SQL语句，实现递归查询
Select * from …. Where [结果过滤条件语句]
Start with [and起始条件过滤语句]
Connect by prior [and中间记录过滤条件语句]
一、Start with.........Connect By 子句
START WITH specifies the root row(s) of the hierarchy.
CONNECT BY specifies the relationship between parent rows and child rows of the hierarchy. In a hierarchical query, one expression in condition must be qualified with the PRIOR operator to refer to the parent row。
二、示例
1. 创建测试表：
CREATE TABLE DG_Test ( D_ID NUMBER, D_NAME VARCHAR2(100 BYTE), D_PID NUMBER , D_Enabled NUMBER )
2. 插入测试数据：
INSERT NTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(1,'10',0,1); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(2,'11',1,1); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(3,'20',0,1); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(4,'12',1,1); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(5,'121',2,1); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(6,'1215',5,0); INSERT INTO dg_test(D_ID,D_NAME,D_PID,D_ENABLED) VALUES(7,'1214',4,0);
3. select * from DG_Test
4. 上述单表情况一般用于维护树型结构的应用
从Root往树末梢递归
select * from dg_test start with D_id=1 connect by prior D_id = D_pid
从末梢往树ROOT递归
select * from dg_test start with D_id=7 connect by D_id = prior D_pid
start with 后面跟着树根，定义了树开始显示的起点位置，如上面两种查询，不管从树根到树顶，还是从树顶到树根。
5. 排除单一体及其分支
select t.* from dg_test t start with D_id=1 connect by prior D_id = D_pid and d_id != 2
这个语句表示从D_id为2的开始，包括后面的子分支，全部排除
注意：这里和where条件的过虑方式不一样，where条件只是把相应的某条不符合要求的记录排除。
