SQL Server 2017

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CREATE TABLE tree  (parent VARCHAR(10), child VARCHAR(10) )

INSERT INTO tree VALUES ('a','b'), ('a','c'), ('b','e'), ('b','f'), ('a','d'), ('b','g'),
('c','h'), ('c','i'), ('d','j'), ('f','k'), ('x','y'), ('y','z'), ('m','n');
13 rows affected

-- SELECT * FROM tree ORDER BY parent, child
WITH rcte (child, parent, rlevel) AS
(
  SELECT parent AS child, NULL AS parent, '0' as lev FROM tree
    WHERE Parent NOT IN (SELECT Child FROM tree)
  UNION ALL

  SELECT o.child, j.parent, '' AS lev FROM tree o
  JOIN tree j ON o.parent >= j.parent
  -- ORDER BY child
)
SELECT DISTINCT * FROM rcte
ORDER BY parent, child

child
parent
rlevel

a
null
0

m
null
0

x
null
0

b
a

c
a

d
a

e
a

f
a

g
a

h
a

i
a

j
a

k
a

n
a

y
a

z
a

e
b

f
b

g
b

h
b

i
b

j
b

k
b

n
b

y
b

z
b

h
c

i
c

j
c

k
c

n
c

y
c

z
c

j
d

k
d

n
d

y
d

z
d

k
f

n
f

y
f

z
f

n
m

y
m

z
m

y
x

z
x

z
y

child	parent	rlevel
a	null	0
m	null	0
x	null	0
b	a
c	a
d	a
e	a
f	a
g	a
h	a
i	a
j	a
k	a
n	a
y	a
z	a
e	b
f	b
g	b
h	b
i	b
j	b
k	b
n	b
y	b
z	b
h	c
i	c
j	c
k	c
n	c
y	c
z	c
j	d
k	d
n	d
y	d
z	d
k	f
n	f
y	f
z	f
n	m
y	m
z	m
y	x
z	x
z	y