try this code .
with resultTable as(
select RT.channel,RT.sumRevenue,LT.[month-start_date],LT.month_end_date,LT.year_month
from (select t.channel,sum(revenue) as sumRevenue,M.month_index from Month_Table M,Transaction_Table T
where t.date_created BETWEEN m.[month-start_date] AND m.month_end_date
group by m.month_index,t.channel) RT Join Month_Table LT on RT.month_index = LT.month_index
)
select * from resultTable
output:

OR use this query
with resultTable as(
select RT.channel,RT.sumRevenue,LT.[month-start_date],LT.month_end_date,LT.year_month
from (select t.channel,sum(revenue) as sumRevenue,M.month_index from Month_Table M,Transaction_Table T
where t.date_created BETWEEN m.[month-start_date] AND m.month_end_date
group by m.month_index,t.channel) RT Join Month_Table LT on RT.month_index = LT.month_index
)
select *,LAG(sumRevenue,1) OVER (PARTITION BY channel ORDER BY channel) previous_month_sales from resultTable
output:
