Zapytanie MySQL - ostatnie wpisy na grupę

Utrzymałbym to naprawdę prosto i użyłbym wyzwalacza do utrzymania last_post_id w tabeli kategorii, dzięki czemu możesz łatwo dołączyć z powrotem do tabeli postów - coś takiego:

Proste zapytanie

select
 pc.cat_id,
 pc.name,
 u.username,
 bp.*
from
 post_category pc
inner join blog_post bp on pc.last_post_id = bp.post_id
inner join users u on bp.user_id = u.user_id
order by
 pc.cat_id;

+--------+------+----------+---------+---------+---------------------+
| cat_id | name | username | post_id | user_id | post_date           |
+--------+------+----------+---------+---------+---------------------+
|      1 | cat1 | bar      |       3 |       2 | 2011-02-09 12:45:33 |
|      2 | cat2 | BAR      |       5 |       3 | 2011-02-09 12:45:33 |
|      3 | cat3 | f00      |       4 |       1 | 2011-02-09 12:45:33 |
+--------+------+----------+---------+---------+---------------------+

Stoły

drop table if exists post_category;
create table post_category
(
cat_id smallint unsigned not null auto_increment primary key,
name varchar(255) unique not null,
last_post_id int unsigned null,
key (last_post_id)
)
engine=innodb;

drop table if exists users;
create table users
(
user_id int unsigned not null auto_increment primary key,
username varbinary(32) unique not null
)
engine=innodb;

drop table if exists blog_post;
create table blog_post
(
post_id int unsigned not null auto_increment primary key,
user_id int unsigned not null,
post_date datetime not null,
key (post_date, user_id)
)
engine=innodb;

drop table if exists blog_post_category;
create table blog_post_category
(
cat_id smallint unsigned not null,
post_id int unsigned not null,
primary key (cat_id, post_id)
)
engine=innodb;

Wyzwalacze

delimiter #

create trigger blog_post_before_ins_trig before insert on blog_post
for each row
begin
  set new.post_date = now();
end#

create trigger blog_post_category_before_ins_trig before insert on blog_post_category
for each row
begin
  update post_category set last_post_id = new.post_id where cat_id = new.cat_id;
end#

delimiter ;

Dane testowe

insert into post_category (name) values ('cat1'),('cat2'),('cat3'),('cat4');
insert into users (username) values ('f00'),('bar'),('BAR'),('alpha'),('beta');

insert into blog_post (user_id) values (1),(1),(2),(1),(3);
insert into blog_post_category (cat_id, post_id) values
(1,1),(1,3),
(2,1),(2,5),
(3,1),(3,3),(3,4);

Mam nadzieję, że to pomoże :)