how to get difference in two rows for a column field in sql

Hello

I have a table

create table #bp_patient65(id int identity(1,1),patient_id int,bp_type varchar(50),encounter_id int,BP varchar(50))

insert into #bp_patient65 values(64874,'Systolic',146019,150),(64874,'Systolic',146020,130),
(64875,'Systolic',146022,150),(64876,'Systolic',146025,150),
(64876,'Systolic',146026,130),(64877,'Systolic',146028,150)

select * from #bp_patient65
drop table #bp_patient65

i want my output like that

patient_id----   bp_type------ encounter_id----- BP--------diff
64874---------- Systolic-----   146019--------  150-----    0
64874---------- Systolic-----   146020--------  130-----    20
64875---------- Systolic-----   146022--------  150-----    0
64876---------- Systolic-----   146025--------  150-----    0
64876---------- Systolic-----   146026--------  130-----    20
64877---------- Systolic-----   146028--------  150-----    0

SELECT patient_id, bp_type,encounter_id,BP,
       COALESCE(
       (
       SELECT TOP 1 BP
       FROM #bp_patient65 mi
       WHERE mi.patient_id > m.patient_id
       group by BP,mi.patient_id
       ORDER BY  patient_id
       ), 0) - BP AS diff
FROM  #bp_patient65 m
group by patient_id,bp_type,encounter_id,BP
ORDER BY
      patient_id