试试这个：

SELECT total, total_1, MONTHNAME(costs_1.updated_at) from costs_1 INNER JOIN costs_2 
ON 
MONTH(costs_1.updated_at)=MONTH(costs_2.updated_at)

我相信你想要这样的东西。

SELECT t1.y year,
       t1.m month,
       t1.total,
       t2.total
FROM
(
    SELECT YEAR(updated_at) y, 
           MONTH(updated_at) m, 
           SUM(total) total
    FROM costs_1
    GROUP BY YEAR(updated_at), MONTH(updated_at) 
) t1
JOIN
(
    SELECT YEAR(updated_at_1) y, 
           MONTH(updated_at_1) m, 
           SUM(total_1) total
    FROM costs_2
    GROUP BY YEAR(updated_at_1), MONTH(updated_at_1) 
) t2 ON t1.y = t2.y and t1.m = t2.m

但是，在这个解决方案中，如果 costs_1 或者 costs_2 没有任何月份记录。如果您想要完全连接，您必须在mysql中使用两个外部连接来模拟它，如下所示：

SELECT t1.y year,
       t1.m month,
       COALESCE(t1.total, 0),
       COALESCE(t2.total, 0)
FROM
(
    SELECT YEAR(updated_at) y, 
           MONTH(updated_at) m, 
           SUM(total) total
    FROM costs_1
    GROUP BY YEAR(updated_at), MONTH(updated_at) 
) t1
LEFT JOIN
(
    SELECT YEAR(updated_at_1) y, 
           MONTH(updated_at_1) m, 
           SUM(total_1) total
    FROM costs_2
    GROUP BY YEAR(updated_at_1), MONTH(updated_at_1) 
) t2 ON t1.y = t2.y and t1.m = t2.m

   UNION

SELECT t2.y year,
       t2.m month,
       COALESCE(t1.total, 0),
       COALESCE(t2.total, 0)
FROM
(
    SELECT YEAR(updated_at) y, 
           MONTH(updated_at) m, 
           SUM(total) total
    FROM costs_1
    GROUP BY YEAR(updated_at), MONTH(updated_at) 
) t1
RIGHT JOIN
(
    SELECT YEAR(updated_at_1) y, 
           MONTH(updated_at_1) m, 
           SUM(total_1) total
    FROM costs_2
    GROUP BY YEAR(updated_at_1), MONTH(updated_at_1) 
) t2 ON t1.y = t2.y and t1.m = t2.m