当你按 MONTH 分组时，它也应该检查 YEAR 总是好的

select SUM(transaction) as Price, 
       DATE_FORMAT(transDate, "%m-%Y") as 'month-year' 
       from transaction group by DATE_FORMAT(transDate, "%m-%Y");

对于 SQLITE

select SUM(transaction) as Price, 
       strftime("%m-%Y", transDate) as 'month-year' 
       from transaction group by strftime("%m-%Y", transDate);