一.建立表

DECLARE @t Table(id CHAR(1),parentId CHAR(1))
INSERT INTO @t values('a','')
INSERT INTO @t values('b','a')
INSERT INTO @t values('c','b')
INSERT INTO @t values('d','c')
INSERT INTO @t values('e','')
INSERT INTO @t values('f','e')
INSERT INTO @t values('g','f')

表中資料：

SELECT * FROM @t
/*
id    parentId
a     
b    a
c    b
d    c
e     
f    e
g    f
*/
 

二.查詢首節點和尾節點

1）首節點

--首節點
SELECT * FROM @t A WHERE NOT EXISTS(
SELECT * FROM @t B WHERE a.parentId=b.id
) 
/*
id    parentId
a     
e     
*/

2）尾節點

--尾節點
SELECT * FROM @t A WHERE NOT EXISTS(
SELECT * FROM @t B WHERE a.id=b.parentId
) 
/*
id    parentId
d    c
g    f
*/