Oracle 18c

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with 
	variables_data as ( 
		select 1 variables_id, 'maintenance_price' variables_name, 30.00 variables_value, timestamp'2020-08-01 05:00:00.000' variables_updated_at from dual
	),
	variables_history_data as (
		select 1 variables_id, 'maintenance_price' variables_name, 15.90 variables_value, timestamp'2019-10-01 11:30:00.000' variables_hist_updated_at from dual union all
		select 1 variables_id, 'maintenance_price' variables_name, 10.50 variables_value, timestamp'2020-01-01 01:00:00.000' variables_hist_updated_at from dual union all
		select 1 variables_id, 'maintenance_price' variables_name, 20.30 variables_value, timestamp'2020-05-01 12:30:00.000' variables_hist_updated_at from dual
	),
	maintenances as (
		select 1 maintenance_id, 'COMPLETE' maintenance_status, timestamp'2019-02-01 00:30:00.000' maintenance_date from dual union all
		select 2 maintenance_id, 'COMPLETE' maintenance_status, timestamp'2019-05-01 01:30:00.000' maintenance_date from dual union all
		select 3 maintenance_id, 'COMPLETE' maintenance_status, timestamp'2019-11-01 02:30:00.000' maintenance_date from dual union all
		select 4 maintenance_id, 'COMPLETE' maintenance_status, timestamp'2020-07-10 05:30:00.000' maintenance_date from dual union all
		select 5 maintenance_id, 'FAILED' maintenance_status,   timestamp'2020-08-02 11:30:00.000' maintenance_date from dual union all
		select 6 maintenance_id, 'COMPLETE' maintenance_status, timestamp'2020-08-20 11:30:00.000' maintenance_date from dual
	)
select 
    m.maintenance_id,
    to_char(m.maintenance_date, 'yyyy/mm/dd') as maintenance_date,
    coalesce(v.variables_value, vh.variables_value) as variables_value
from maintenances m
outer apply(
	select v.variables_value
	from variables_data v
	where v.variables_updated_at <= m.maintenance_date
) v
outer apply (
    select vh.variables_value
	from variables_history_data vh
	where vh.variables_hist_updated_at > m.maintenance_date
	order by vh.variables_hist_updated_at
    fetch first 1 row only
) vh
where m.maintenance_status = 'COMPLETE'
order by 1;

MAINTENANCE_ID	MAINTENANCE_DATE	VARIABLES_VALUE
1	2019/02/01	15.9
2	2019/05/01	15.9
3	2019/11/01	10.5
4	2020/07/10	null
6	2020/08/20	30