表结构

我们的需求是：取出每个项目中最大上项目时间对应的那条数据。（即根据project_id分组，取出每组中最大的on_project_time对应的数据。）上方红框是我们要查出的数据。

sql如下 

select t1.*

FROM project_record t1

INNER JOIN (SELECT SUBSTRING_INDEX(GROUP_CONCAT(id ORDER BY on_project_time DESC),',',1) AS id

FROM project_record GROUP BY project_id) AS t2 ON t2.id = t1.id;

 查询重复数据

SELECT column1, column2, COUNT(*) as count FROM table_name GROUP BY column1, column2 HAVING COUNT(*) > 1;