SQL Server 2017

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create table your_table (SDO int,Startmonth varchar(3),endmonth varchar(3),  rate int);
insert into your_table
values
(101,'jan','mar',10),
(101,'feb','mar',20),
(101,'may','jul',30);
3 rows affected

Select *
from
(
	select 
	case when tab2.SDO is null then tab1.SDO else tab2.SDO end as SDO,
	case when tab2.Startmonth is null then tab1.Startmonth else tab2.Startmonth end as Startmonth,
	case when tab2.endmonth is null then tab1.endmonth else tab2.endmonth end as endmonth,
	case when tab2.rate is null then tab1.rate else tab2.rate end as rate
	from your_table tab1
	left join your_table tab2
	on (tab2.Startmonth > tab1.Startmonth
		and tab2.Startmonth < tab1.endmonth ) or
		tab2.Startmonth is null
) as tab
group by SDO,Startmonth,endmonth,rate
having count(*) = 1


SDO
Startmonth
endmonth
rate


101
may
jul
30