根据查询结果更新

merge into … using…

merge into h
using m
on (h.garage_contract_main_id = m.garage_contract_main_id and h.garage_id = m.garage_id)
when matched then
  update
     set h.contract_start_date = m.contract_start_date,
         h.contract_end_date   = m.contract_end_date,
         h.business_start_date = m.business_start_date,
         h.business_end_date   = m.business_end_date,
         h.file_flag           = m.file_flag,
         h.effective           = m.effective
   where m.contract_start_date is not null
     and h.contract_start_date is null
     and m.statu = '1';

解释：
在h表中，找到与m表中具有相同garage_contract_main_id和garage_id的记录，并且只有当m表中的contract_start_date不为空，而h表中的contract_start_date为空，且m表中的statu字段值为’1’时，才更新h表中的contract_start_date、contract_end_date、business_start_date和business_end_date字段