Need Help with Hierarchical Mysql Query

Question

I\'m trying to wrap my head around querying a table that has hierarchical category data (used for a cms) which is also tied to my posts data and a many-to-many type relation

Answer 1

There are perhaps better methods, but here's an idea:

CREATE VIEW subcategories AS
( SELECT 0        AS level
       , cat_0.id AS cat_id
       , cat_0.id AS subcat_id
  FROM categories cat_0

  UNION ALL

  SELECT 1        AS level
       , cat_0.id AS cat_id
       , cat_1.id AS subcat_id
  FROM categories cat_0
    JOIN categories cat_1
      ON cat_0.id = cat_1.parent_id

  UNION ALL

  SELECT 2        AS level
       , cat_0.id AS cat_id
       , cat_2.id AS subcat_id
  FROM categories cat_0
    JOIN categories cat_1
      ON cat_0.id = cat_1.parent_id
    JOIN categories cat_2
      ON cat_1.id = cat_2.parent_id

  UNION ALL

  SELECT 3        AS level
       , cat_0.id AS cat_id
       , cat_3.id AS subcat_id
  FROM categories cat_0
    JOIN categories cat_1
      ON cat_0.id = cat_1.parent_id
    JOIN categories cat_2
      ON cat_1.id = cat_2.parent_id
    JOIN categories cat_3
      ON cat_2.id = cat_3.parent_id
) ;

Then, use the above to have:

SELECT sub.subcat_id
     , sub.level
     , p.post_id
     , p.title
FROM subcategories sub
  JOIN post2cat p2c
    ON sub.subcat_id = p2c.cat_id
  JOIN posts p
    ON p2c.post_id = p.id
WHERE sub.cat_id = CAT_ID        <--- the category id you want to search for

Answer 2

It is a challenge to fetch the tree to an arbitrary depth n using just SQL in MySQL (See: Managing Hierarchical Data in MySQL and Hierarchical Queries in MySQL).

If you are dealing with a small dataset, it makes the most sense to select all of the categories from the database and then build the tree in a scripting language. Then the tree can be traversed to find parent node and the parent node's children, which can then be used to query the database for the posts using a join.

Answer 3

The following stored procedure should provide a good starting point or provide some inspiration.

Hope it helps :)

Example calls

You can call the stored procedure from php as follows:

$result = $conn->query(sprintf("call category_hier(%d)", 1));

or from the mysql command line:

mysql> call category_hier(1);
+--------+---------------+---------------+----------------------+-------+
| cat_id | category_name | parent_cat_id | parent_category_name | depth |
+--------+---------------+---------------+----------------------+-------+
|      1 | Location      |          NULL | NULL                 |     0 |
|      3 | USA           |             1 | Location             |     1 |
|      4 | Illinois      |             3 | USA                  |     2 |
|      5 | Chicago       |             3 | USA                  |     2 |
+--------+---------------+---------------+----------------------+-------+
4 rows in set (0.00 sec)

Full script

drop table if exists categories;
create table categories
(
cat_id smallint unsigned not null auto_increment primary key,
name varchar(255) not null,
parent_cat_id smallint unsigned null,
key (parent_cat_id)
)
engine = innodb;

insert into categories (name, parent_cat_id) values
('Location',null), 
('Color',null), 
   ('USA',1), 
      ('Illinois',3), 
      ('Chicago',3), 
   ('Black',2), 
   ('Red',2);


drop procedure if exists category_hier;
delimiter #

create procedure category_hier
(
in p_cat_id smallint unsigned
)
begin

declare v_done tinyint unsigned default 0;
declare v_depth smallint unsigned default 0;

create temporary table hier(
 parent_cat_id smallint unsigned, 
 cat_id smallint unsigned, 
 depth smallint unsigned default 0
)engine = memory;

insert into hier select parent_cat_id, cat_id, v_depth from categories where cat_id = p_cat_id;
create temporary table tmp engine=memory select * from hier;

/* http://dev.mysql.com/doc/refman/5.0/en/temporary-table-problems.html */

while not v_done do

    if exists( select 1 from categories c
        inner join tmp on c.parent_cat_id = tmp.cat_id and tmp.depth = v_depth) then

        insert into hier select c.parent_cat_id, c.cat_id, v_depth + 1 from categories c
            inner join tmp on c.parent_cat_id = tmp.cat_id and tmp.depth = v_depth;

        set v_depth = v_depth + 1;          

        truncate table tmp;
        insert into tmp select * from hier where depth = v_depth;

    else
        set v_done = 1;
    end if;

end while;

select 
 c.cat_id,
 c.name as category_name,
 p.cat_id as parent_cat_id,
 p.name as parent_category_name,
 hier.depth
from 
 hier
inner join categories c on hier.cat_id = c.cat_id
left outer join categories p on hier.parent_cat_id = p.cat_id
order by
 hier.depth;

drop temporary table if exists hier;
drop temporary table if exists tmp;

end #

delimiter ;

call category_hier(1);

call category_hier(2);