SQL 按月分组查询，统计，耗时，精确到天

SELECT
sum( spendTime ) spendTime,
DATE_FORMAT ( create_time, '%Y-%m' ) monthTime
FROM
(
SELECT
a.CODE,
a.approve_initiator_time,
a.approve_time,
round( TIMESTAMPDIFF( HOUR, a.approve_initiator_time, a.approve_time ) / 24, 1 ) spendTime,
c.create_time
FROM
contract c
INNER JOIN approve a ON c.contract_no = a.CODE
AND c.is_delete = '0'

) t
GROUP BY
DATE_FORMAT ( t.create_time, '%Y-%m' );