【SQL查詢】樹結構查詢

條件1: 表示從哪一個節點開始查找, 也就是經過條件1 查詢到的數據, 做爲後續查詢的起始節點(參數).
條件2: 是鏈接條件，其中用PRIOR表示上一條記錄，例如CONNECT BY PRIOR ID = PID，意思就是上一條記錄的ID是本條記錄的PID，即本記錄的父親是上一條記錄。
條件3: 不能用在 Connect By 後, 這裏的條件判斷, 等價於 在最後查詢出結果列表以後, 再進行條件篩選; 並不是 刪除掉 節點及子節點;

關鍵點：
nocycle關鍵字, 有時候數據自己 不合理會致使出現循環的問題, 如 將上述的 ID '00001' 記錄的 'PID' 也改成 '00001', 會出現循環的問題, 這是, 須要用到 nocycle 便可消除循環;
Connect By nocycle Prior ID = PID 便可.
connect_by_isleaf 表示當前節點是不是葉子節點
level 表示當前節點所處層級, 這裏的層級指的是 從 start with 查詢到的節點開始往下算起, 當前屬於第幾層級

示例：
 (1) 腳本格式：
SELECT ... FROM + 表名 WHERE + 條件3 START WITH + 條件1 CONNECT BY PRIOR + 條件2

/* 表格 */
create table tb_menu(
	id number(10) not null,
	title varchar(50),
	parent number(10)
)
--父菜單
insert into tb_menu(id, title, parent) values(1, '父菜單1',null);
insert into tb_menu(id, title, parent) values(2, '父菜單2',null);
insert into tb_menu(id, title, parent) values(3, '父菜單3',null);
insert into tb_menu(id, title, parent) values(4, '父菜單4',null);
insert into tb_menu(id, title, parent) values(5, '父菜單5',null);
--一級菜單
insert into tb_menu(id, title, parent) values(6, '一級菜單6',1);
insert into tb_menu(id, title, parent) values(7, '一級菜單7',1);
insert into tb_menu(id, title, parent) values(8, '一級菜單8',1);
insert into tb_menu(id, title, parent) values(9, '一級菜單9',2);
insert into tb_menu(id, title, parent) values(10, '一級菜單10',2);
insert into tb_menu(id, title, parent) values(11, '一級菜單11',2);
insert into tb_menu(id, title, parent) values(12, '一級菜單12',3);
insert into tb_menu(id, title, parent) values(13, '一級菜單13',3);
insert into tb_menu(id, title, parent) values(14, '一級菜單14',3);
insert into tb_menu(id, title, parent) values(15, '一級菜單15',4);
insert into tb_menu(id, title, parent) values(16, '一級菜單16',4);
insert into tb_menu(id, title, parent) values(17, '一級菜單17',4);
insert into tb_menu(id, title, parent) values(18, '一級菜單18',5);
insert into tb_menu(id, title, parent) values(19, '一級菜單19',5);
insert into tb_menu(id, title, parent) values(20, '一級菜單20',5);
--二級菜單
insert into tb_menu(id, title, parent) values(21, '二級菜單21',6);
insert into tb_menu(id, title, parent) values(22, '二級菜單22',6);
insert into tb_menu(id, title, parent) values(23, '二級菜單23',7);
insert into tb_menu(id, title, parent) values(24, '二級菜單24',7);
insert into tb_menu(id, title, parent) values(25, '二級菜單25',8);
insert into tb_menu(id, title, parent) values(26, '二級菜單26',9);
insert into tb_menu(id, title, parent) values(27, '二級菜單27',10);
insert into tb_menu(id, title, parent) values(28, '二級菜單28',11);
insert into tb_menu(id, title, parent) values(29, '二級菜單29',12);
insert into tb_menu(id, title, parent) values(30, '二級菜單30',13);
insert into tb_menu(id, title, parent) values(31, '二級菜單31',14);
insert into tb_menu(id, title, parent) values(32, '二級菜單32',15);
insert into tb_menu(id, title, parent) values(33, '二級菜單33',16);
insert into tb_menu(id, title, parent) values(34, '二級菜單34',17);
insert into tb_menu(id, title, parent) values(35, '二級菜單35',18);
insert into tb_menu(id, title, parent) values(36, '二級菜單36',19);
insert into tb_menu(id, title, parent) values(37, '二級菜單37',20);
--三級菜單
insert into tb_menu(id, title, parent) values(38, '三級菜單38',21);
insert into tb_menu(id, title, parent) values(39, '三級菜單39',22);
insert into tb_menu(id, title, parent) values(40, '三級菜單40',23);
insert into tb_menu(id, title, parent) values(41, '三級菜單41',24);
insert into tb_menu(id, title, parent) values(42, '三級菜單42',25);
insert into tb_menu(id, title, parent) values(43, '三級菜單43',26);
insert into tb_menu(id, title, parent) values(44, '三級菜單44',27);
insert into tb_menu(id, title, parent) values(45, '三級菜單45',28);
insert into tb_menu(id, title, parent) values(46, '三級菜單46',28);
insert into tb_menu(id, title, parent) values(47, '三級菜單47',29);
insert into tb_menu(id, title, parent) values(48, '三級菜單48',30);
insert into tb_menu(id, title, parent) values(49, '三級菜單49',31);
insert into tb_menu(id, title, parent) values(50, '三級菜單50',31);
commit;

(2) select level, t.* from tb_menu t start with id = 1 connect by prior t.id = t.parent;

(3)select level, t.id, lpad(' ', level * 4) || t.title as title, t.parent
  from tb_menu t
 start with id = 6
connect by prior t.id = t.parent;

id=6爲起始節點，上一條記錄的id是本記錄的parent。也稱爲：本記錄的parent是上一條記錄的id

(4)select level, t.* from tb_menu t start with id = 6 connect by t.id = prior t.parent;

id=6爲起始節點，上一條記錄的parent是本記錄的id。也稱爲：本記錄的id是上一條記錄的parent