SQL Server 2017

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select *
into t
from (values 
             (1, 50),
             (2, 50),
             (3, 62),
             (4, 62),
             (5, 62),
             (6, 79),
             (7, 90),
             (8, 90)
            ) v(id, value)
8 rows affected

select t.*, tprev.value
from t outer apply
     (select top (1) tprev.*
      from t tprev
      where tprev.value <> t.value and
            tprev.id < t.id
      order by tprev.id desc
     ) tprev;


id
value
value


1
50
null

2
50
null

3
62
50

4
62
50

5
62
50

6
79
62

7
90
79

8
90
79


select t.*,
       max(case when prev_value <> value then prev_value end) over (partition by grp) as prev_value
from (select t.*,
             sum(case when prev_value = value then 0 else 1 end) over (order by id) as grp
      from (select t.*,
                   lag(value) over (order by id) as prev_value
            from t
           ) t
     ) t


id
value
prev_value
grp
prev_value


1
50
null
1
null

2
50
50
1
null

3
62
50
2
50

4
62
62
2
50

5
62
62
2
50

6
79
62
3
62

7
90
79
4
79

8
90
90
4
79


Warning: Null value is eliminated by an aggregate or other SET operation.

id	value	prev_value	grp	prev_value
1	50	null	1	null
2	50	50	1	null
3	62	50	2	50
4	62	62	2	50
5	62	62	2	50
6	79	62	3	62
7	90	79	4	79
8	90	90	4	79