層次查詢（分級查詢）

一般格式：

SELECT [LEVEL], 列名, 表示式 ... FROM 表格 [WHERE 條件] [START WITH 條件] [CONNECT BY PRIOR 條件];

--從頂向下查詢
1.查詢KING管理的所有的下級員工資訊
SELECT * FROM emp START WITH ename = 'KING' CONNECT BY PRIOR 父列 = 子列;--empno父列   mgr子列
SELECT * FROM emp START WITH ename = 'KING' CONNECT BY PRIOR empno = mgr;
SELECT
 
 * FROM emp START WITH ename = 'KING' CONNECT BY mgr = PRIOR empno;--同上

--從下向上查詢
1.查詢SMITH的所有的上級員工資訊
SELECT * FROM emp START WITH ename = 'SMITH' CONNECT BY PRIOR 子列 = 父列;--empno父列   mgr 列  
SELECT * FROM emp START WITH ename = 'SMITH' CONNECT BY PRIOR mgr = empno;
SELECT * FROM emp START WITH ename =
 
 'SMITH' CONNECT BY empno = PRIOR mgr;--同上

--使用LEVEL偽列
SELECT LEVEL,emp.* FROM emp START WITH ename = 'KING' CONNECT BY PRIOR empno = mgr;
SELECT LEVEL,emp.* FROM emp START WITH ename = 'SMITH' CONNECT BY PRIOR mgr = empno;

--使用LEVEL，以及LPAD生成樹狀報告
SELECT LEVEL,ename, LPAD(ename, LENGTH(ename) + LEVEL
 
*8, ' ') FROM emp START WITH empno = 7839 CONNECT BY PRIOR empno = mgr;

--排除某幾行資料：使用WHERE新增限制條件
1.查詢7839管理的所有下級員工資訊，但是不包括SCOTT的資訊
SELECT LEVEL, ename, LPAD(ename, LENGTH(ename) + LEVEL * 8, ' ')
  FROM emp WHERE ename <> 'SCOTT'
 START WITH empno = 7839
CONNECT BY PRIOR empno = mgr;
--使用CONNECT BY新增限制條件
2.查詢7839管理的所有下級員工資訊，但是不包括BLAKE以及BLAKE所有下屬的資訊
SELECT LEVEL, ename, LPAD(ename, LENGTH(ename) + LEVEL * 8, ' ')
  FROM emp
 START WITH empno = 7839
CONNECT BY PRIOR empno = mgr AND ename <> 'BLAKE';