FWIW in Ecto master there’s support for the filter expressions. Using them, this could be achieved with a simpler query:
{account, inflow, outflow} =
Repo.one(from a in App.Account,
left_join: t in App.Transaction,
on: t.account_id == a.id and t.user_id == ^current_user.id and not t.deleted,
where: a.id == ^id,
group_by: a.id,
select: {a, filter(sum(t.amount), t.type == "inflow"), filter(sum(t.amount), t.type == "outflow")})
