假设 on C.id on A.a 是一个拼写错误，而且是真的 on C.id = A.a :

select a.a, a.b, c.c
from inventory a 
left join 
(
  select b.a as id, b.a * 100 as c
  from inventory b
  where condition1 and condition2
) as c on c.id = a.a;

等于

select a.a, a.b, 
  (
    select b.a * 100
    from inventory b
    where condition1 and condition2
    and b.a = a.a
  ) as c
from inventory a;

cosmos db目前只支持自连接。您可以参考此文档。左连接在cosmos db开发团队计划中，暂定于2020年下半年开始。您可以在此处跟踪此功能。