SO 46988030 (2), MySQL

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drop table if exists t1;

create table t1 (id int, title varchar(100), category varchar(100), time_stamp datetime);
insert into t1 values 
  ( 1, "post 1", "category 1", "2017-10-28 14:03"),
  ( 2, "post 2", "category 1", "2017-10-28 14:05"),
  ( 3, "post 3", "category 4", "2017-10-28 14:05"),
  ( 4, "post 4", "category 1", "2017-10-28 14:08"),
  ( 5, "post 5", "category 2", "2017-10-28 14:09"),
  ( 6, "post 6", "category 5", "2017-10-28 14:13"),
  ( 7, "post 7", "category 3", "2017-10-28 14:14"),
  ( 8, "post 8", "category 6", "2017-10-28 14:16:30"),
  ( 9, "post 9", "category 2", "2017-10-28 14:16"),
  (10, "post 10", "category 2", "2017-10-28 14:16:30"),
  (11, "post 11", "category 4", "2017-10-28 14:18"),
  (12, "post 12", "category 4", "2017-10-28 14:18");

select * from
t1
where (id,category,time_stamp) in (select
                                   max(id),category,max(time_stamp)
                                from t1
                                group by category)
group by category
order by time_stamp desc
limit 3
;

drop table t1;


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