Question

oracle中的SQL的connect by是做什么用的

Answer 1

树结构，同时要跟START WITH一起用

Answer 2

--用法一
[TEST@orcl] SQL>select rownum from dual connect by rownum < 4;
ROWNUM
----------
1
2
3
--用法二
[SCOTT@orcl] SQL>SELECT EMPNO,ENAME,MGR
2 　　FROM EMP
3 　　CONNECT BY PRIOR EMPNO=MGR
4 　　START WITH ENAME='BLAKE';

EMPNO ENAME MGR
---------- ---------- ----------
7698 BLAKE 7839
7499 ALLEN 7698
7521 WARD 7698
7654 MARTIN 7698
7844 TURNER 7698
7900 JAMES 7698

Answer 3

一般是用来查询 数型结构的。
例如：
CREATE TABLE test_tree (
test_id INT,
pid INT,
test_val VARCHAR(10),
PRIMARY KEY (test_id)
);

INSERT INTO test_tree VALUES(1, NULL, '.NET');
INSERT INTO test_tree VALUES(2, 1, 'C#');
INSERT INTO test_tree VALUES(3, 1, 'J#');
INSERT INTO test_tree VALUES(4, 1, 'ASP.NET');
INSERT INTO test_tree VALUES(5, 1, 'VB.NET');

INSERT INTO test_tree VALUES(6, NULL, 'J2EE');
INSERT INTO test_tree VALUES(7, 6, 'EJB');
INSERT INTO test_tree VALUES(8, 6, 'Servlet');
INSERT INTO test_tree VALUES(9, 6, 'JSP');

INSERT INTO test_tree VALUES(10, NULL, 'Database');
INSERT INTO test_tree VALUES(11, 10, 'DB2');
INSERT INTO test_tree VALUES(12, 10, 'MySQL');
INSERT INTO test_tree VALUES(13, 10, 'Oracle');
INSERT INTO test_tree VALUES(14, 10, 'SQL Server');

INSERT INTO test_tree VALUES(15, 13, 'PL/SQL');
INSERT INTO test_tree VALUES(16, 15, 'Function');
INSERT INTO test_tree VALUES(17, 15, 'Procedure');
INSERT INTO test_tree VALUES(18, 15, 'Package');
INSERT INTO test_tree VALUES(19, 15, 'Cursor');

INSERT INTO test_tree VALUES(20, 14, 'T-SQL');

SQL> ed
Wrote file afiedt.buf

1 SELECT
2 LPAD(' ', 2*(LEVEL-1)) || test_val AS test_val
3 FROM
4 test_tree
5 START WITH
6 test_id IN (1, 6, 10)
7* CONNECT BY PRIOR test_id = pid
SQL> /

TEST_VAL
-----------------------------------------------------------

.NET
C#
J#
ASP.NET
VB.NET
J2EE
EJB
Servlet
JSP
Database
DB2

TEST_VAL
-----------------------------------------------------------

MySQL
Oracle
PL/SQL
Function
Procedure
Package
Cursor
SQL Server
T-SQL

20 rows selected.