SQL Server 2016

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id	mark_A	mark_B	real_group
10	mark_1	mark_2	1
24	mark_1	mark_2	1
29	mark_1	mark_152	1
30	mark_152	mark_1	1
31	mark_152	mark_300	1
32	mark_200	mark_250	1
33	mark_250	mark_152	1
25	mark_3	mark_4	2
26	mark_3	mark_5	2
27	mark_4	mark_3	2
28	mark_5	mark_6	2
11	mark_3	mark_4	2
12	mark_3	mark_5	2
13	mark_4	mark_3	2
14	mark_5	mark_6	2
15	mark_7	mark_3	2
16	mark_7	mark_18	2
17	mark_18	mark_19	2
18	mark_8	mark_9	3
19	mark_10	mark_11	3
20	mark_10	mark_8	3
21	mark_12	mark_13	4
22	mark_14	mark_15	5
23	mark_16	mark_17	6

with marksOneLine as 
(
	select mark_A as mark from #all_marks
	union
	select mark_B as mark from #all_marks
)
, marksPairs as
(
	select mark_A, mark_B from #all_marks where mark_A <> mark_B
	union
	select mark_B as mark_A, mark_A as mark_B from #all_marks where mark_A <> mark_B
)
, marksRecursive as
(
	select
		cast(marksOneLine.mark as varchar(8000)) as anchorMark
		, mark_A
		, mark_B
		, cast(',' + mark_A + ',' + mark_B + ',' as varchar(8000)) as markPath
		, 1 as lvl
	from 
		marksPairs
		inner join marksOneLine on marksOneLine.mark = marksPairs.mark_A
	union all
	select
		marksRecursive.anchorMark
		, marksPairs.mark_A
		, marksPairs.mark_B
		, cast(marksRecursive.markPath + marksPairs.mark_B + ',' as varchar(8000)) as markPath
		, marksRecursive.lvl + 1 as lvl
	from marksPairs
	inner join marksRecursive on marksRecursive.mark_B = marksPairs.mark_A
	where marksRecursive.markPath not like cast ('%,' + marksPairs.mark_B + ',%' as varchar(8000))
)
, marksRecursionResult as
(
	select anchorMark, mark_A, mark_B
	from marksRecursive
)
, marksCleanResult as
(
	select anchorMark, mark_A as mark from marksRecursionResult
	union
	select anchorMark, mark_B as mark from marksRecursionResult
)
, marksResult as 
(
	select
		marksOneLine.mark
		, case when CA_Data.XML_Value is null then marksOneLine.mark else CA_Data.XML_Value end as groupMembers
		, DENSE_RANK() over(order by case when CA_Data.XML_Value is null then marksOneLine.mark else CA_Data.XML_Value end) as groupId
	from 
		marksOneLine
		cross apply
		(
			select marksCleanResult.mark + ','
			from marksCleanResult
			where marksCleanResult.anchorMark = marksOneLine.mark
			order by marksCleanResult.mark for XML PATH(''), type
		) as CA_XML(XML_VALUE)
		cross apply
		(
			select CA_XML.XML_VALUE.value('.', 'NVARCHAR(MAX)')
		) as CA_Data(XML_VALUE)
	where
		marksOneLine.mark is not null
)
select a.*, r.groupId
from #all_marks a
join marksResult r on r.mark = a.mark_A
order by real_group

id	mark_A	mark_B	real_group	groupId
10	mark_1	mark_2	1	1
24	mark_1	mark_2	1	1
29	mark_1	mark_152	1	1
30	mark_152	mark_1	1	1
31	mark_152	mark_300	1	1
32	mark_200	mark_250	1	1
33	mark_250	mark_152	1	1
25	mark_3	mark_4	2	6
26	mark_3	mark_5	2	6
27	mark_4	mark_3	2	6
28	mark_5	mark_6	2	6
11	mark_3	mark_4	2	6
12	mark_3	mark_5	2	6
13	mark_4	mark_3	2	6
14	mark_5	mark_6	2	6
15	mark_7	mark_3	2	6
16	mark_7	mark_18	2	6
17	mark_18	mark_19	2	6
18	mark_8	mark_9	3	2
19	mark_10	mark_11	3	2
20	mark_10	mark_8	3	2
21	mark_12	mark_13	4	3
22	mark_14	mark_15	5	4
23	mark_16	mark_17	6	5