SQL query to check if the next row value is same or different - sql

Question

I am joining two tables based on a common column date. However, the column I am trying to get from one the table (cmg) in this case, should get next row value only if it is different from its previous row's value
Table A
Date comp.no
-----------------------
2019-03-08 5
2019-02-26 5
2019-01-17 5
2019-01-10 5
2018-12-27 5
Table B
Date cmg
-----------------
2019-07-17 NULL
2019-04-20 NULL
2019-02-26 RHB
2019-01-19 NULL
2019-01-17 RHB
2019-01-10 RMB
2018-12-28 NULL
2018-12-27 RHB
2018-12-12 RUB
2018-11-28 RUB
2018-10-20 NULL
2018-07-21 NULL
2018-04-21 NULL
2018-01-20 NULL
2017-10-21 NULL
2017-07-29 NULL
2017-05-07 NULL
2017-02-13 NULL
2016-11-22 NULL
2016-08-29 NULL
2016-06-07 NULL
2016-04-06 RUB
2016-03-21 RUB
2016-03-07 RUB

Answer 1

You can use lag function to compare with previous value. And for the first row you'll need an isnull() check since the first row won't have a previous value.
;with cte as(
select case
when isnull(lag(t2.cmg)over (order by t2.cmg desc),'') <>t2.cmg then 1 else 0 end as isresult
,t2.date,t2.cmg
from TableA t1
inner join TableB t2
on t1.date=t2.date
)
select date,cmg from cte where isresult=1

Answer 2

Use lag():
select date, cmg
from (select b.date, b.cmg, lag(b.cmg) over (order by b.date) as prev_cmg
from a join
b
on a.date = b.date
) b
where prev_cmg is null or prev_cmg <> cmg
order by date;