MySQL Query - recent entries per group - mysql

Question

I'm trying to select the most recent entries per group in a table.
Say I have a table "blog_posts" which has a column for "id" (all unique, auto incremented), "post_cat" which can be values 'category1' or 'category2' or 'category3', and a "publish_status" column which can be values 'online' or 'offline'.
How can I select the most recent entries for each category?
I have the following right now, but it almost feels like it's selecting randomly:
select * FROM `blog_posts` WHERE (publish_status = 'online') GROUP BY post_cat ORDER BY id DESC LIMIT 10

Answer 1

I'd keep it real simple and use a trigger to maintain a last_post_id in the category table so you can easily join back on the posts table - something like this:
Simple Query
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 |
+--------+------+----------+---------+---------+---------------------+
Tables
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;
Triggers
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 ;
Test Data
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);
Hope this helps :)

Answer 2

There's no really straightforward way to go about this...
http://www.artfulsoftware.com/infotree/queries.php#104
http://planet.mysql.com/entry/?id=26926

Answer 3

If you only have three categories, you can just make separate queries and take the union:
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category1' ORDER BY `id` DESC LIMIT 10) UNION
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category2' ORDER BY `id` DESC LIMIT 10) UNION
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category3' ORDER BY `id` DESC LIMIT 10)
You could even re-sort the whole thing at the end. It's only 30 rows!
SELECT * FROM (
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category1' ORDER BY `id` DESC LIMIT 10) UNION
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category2' ORDER BY `id` DESC LIMIT 10) UNION
(SELECT * FROM `blog_posts` WHERE `publish_status` = 'online' AND `post_cat`='category3' ORDER BY `id` DESC LIMIT 10)
) `monster` ORDER BY `id` DESC