mysql关联查询left join获取右表的最新一条

原创于 2022-04-21 14:33:27 发布 · 5k 阅读

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该博客介绍了如何使用SQL的LEFTJOIN结合子查询和GROUP BY来从多个关联记录中筛选出每组最新的记录。通过max和group by函数，可以有效地在`task_supervision`和`task_supervision_child`表之间进行操作，以获取每个`supervisionid`对应的最近创建时间。

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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`