How to Sum columns with sub quires in SQL Server

hi..  i have 4 tables

shipment_assignment      

field = id , member_id,partner_id,vehicle_id,shipment_no, begin_date_time,end_date_time,create_date,active

members      

field = member_name  

vehicles      

field = vehicle_name

trips      

field = vehicle_id , start_date_ time , stop_date_time, distance

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Then I have Design in my view with some join table i want sum distance in  table trips based between begin_date_time and end_date_time IN start_date_time > from trips

SELECT sa.id
      ,sa.shipment_no
      ,v.vehicle_name
      ,m.member_name
      ,sa.begin_date_time
      ,sa.create_date
      ,sa.member_id
      ,sa.partner_id
      ,sa.vehicle_id
      ,sa.end_date_time
      ,sa.active
      ,  CONVERT(VARCHAR(5),DateDiff(dd, sa.begin_date_time, sa.end_date_time)) + ' Days '
       + CONVERT(VARCHAR(5),DateDiff(hh, sa.begin_date_time, sa.end_date_time) % 24) + ' hours '
       + CONVERT(VARCHAR(5),DateDiff(mi, sa.begin_date_time, sa.end_date_time) % 60) + ' minutes '
       + CONVERT(VARCHAR(5),(DATEDIFF(S, sa.begin_date_time, sa.end_date_time)%60))+' seconds' As TotalDiff
FROM dbo.shipment_assignment AS sa INNER JOIN
dbo.vehicles AS v ON v.vehicle_id = sa.vehicle_id INNER JOIN
dbo.members AS m ON sa.partner_id = m.member_id