这应该做到这一点:
select income, expense, income-expense balancefrom (select sum(rate) income from _income) iJOIN (select sum(rate) expense from _expense) e

这应该做到这一点:
select income, expense, income-expense balancefrom (select sum(rate) income from _income) iJOIN (select sum(rate) expense from _expense) e