Mysql，如何按列分组并选择最新日期？

您可以NOT EXISTS按如下方式使用它：

SELECT *
  FROM employment_informations t1
 WHERE NOT EXISTS
       (SELECT 1 FROM employment_informations  t2
         WHERE t1.employee_id = t2.employee_id 
           AND t1.field_name = t2.field_name
           AND t2.created_at > t1.created_at)