关联查询left join 多条记录只保留最新的一条关联。可以通过max+group by实现

SELECT
	`t1`.`id` AS `id`,
	`t1`.`keytasks` AS `keytasks`,
	`t1`.`content` AS `content`,
	`t1`.`company` AS `company`,
	`t1`.`achievegoal` AS `achievegoal`,
	`t1`.`setime` AS `setime`,
	`t4`.`createtime` AS `createtime` 
FROM
	`paastest`.`task_supervision` `t1`
	LEFT JOIN (
	SELECT
		`t1`.`id` AS `id`,
		`t1`.`supervisionid` AS `supervisionid`,
		`t1`.`createtime` AS `createtime` 
	FROM
		(
		SELECT
			`t2`.`id` AS `id`,
			`t2`.`supervisionid` AS `supervisionid`,
			max( `t2`.`createtime` ) AS `createtime` 
		FROM
			`paastest`.`task_supervision_child` `t2` 
		GROUP BY
			`t2`.`supervisionid` 
		) `t1` 
	) t4 ON `t1`.`id` = `t4`.`supervisionid`