MySQL Hierarchical Structure Data Extraction - mysql

Question

I've been struggling for about 2 hours on one query now. Help? :(
I have a table like this:
id name lft rgt
35 Top level board 1 16
37 2nd level board 3 6 15
38 2nd level board 2 4 5
39 2nd level board 1 2 3
40 3rd level board 1 13 14
41 3rd level board 2 9 12
42 3rd level board 3 7 8
43 4th level board 1 10 11
It is stored in the structure recommended in this tutorial. What I want to do is select a forum board and all sub forums ONE level below the selected forum board (no lower). Ideally, the query would get the selected forum's level while only being passed the board's ID, then it would select that forum, and all it's immediate children.
So, I would hopefully end up with:
id name lft rgt
35 Top level board 1 16
37 2nd level board 3 6 15
38 2nd level board 2 4 5
39 2nd level board 1 2 3
Or
id name lft rgt
37 2nd level board 3 6 15
40 3rd level board 1 13 14
41 3rd level board 2 9 12
42 3rd level board 3 7 8
The top rows here are the parent forums, the others sub forums. Also, I'd like something where a depth value is given, where the depth is relative to the selected parent form. For example, taking the last table as some working data, we would have:
id name lft rgt depth
37 2nd level board 3 6 15 0
40 3rd level board 1 13 14 1
41 3rd level board 2 9 12 1
42 3rd level board 3 7 8 1
Or
id name lft rgt depth
35 Top level board 1 16 0
37 2nd level board 3 6 15 1
38 2nd level board 2 4 5 1
39 2nd level board 1 2 3 1
I hope you get my drift here.
Can anyone help with this? It's really getting me annoyed now :(
James

Answer 1

The easiest way for you to do it - just add a column where you keep the depth.
Otherwise the query will be very inefficient - you will have to get a the whole hierarchy, sorted by left number (that will put very first child be first), join it to itself to make sure that for each next node left number is equal to previous node right number + 1
In general, nested intervals algorithm is nice, but has a serious disadvantage - if you add something to tree, a lot of recalculations required.
A nice alternative for this is Tropashko Nested intervals algorithm with continued fractions - just google for it. And getting a single level below the parent with this algorithm is done very naturally. Also, given a child, you can calculate all numbers for all its parents without hitting a database.

Answer 2

One more thing to consider is that relational databases really are not the most optimal and natural way to store hierarchical data. A structure like you have here - a binary tree, essentially - would be much easier to represent with an XML blob that you can persist, or store as an object in an object-oriented database.

Answer 3

I prefer the adjacency list approach myself. The following example uses a non-recursive stored procedure to return a tree/subtree which I then transform into an XML DOM but you could do whatever you like with the resultset. Remember it's a single call from PHP to MySQL and adjacency lists are much easier to manage.
full script here : http://pastie.org/1294143
PHP
<?php
header("Content-type: text/xml");
$conn = new mysqli("localhost", "foo_dbo", "pass", "foo_db", 3306);
// one non-recursive db call to get the tree
$result = $conn->query(sprintf("call department_hier(%d,%d)", 2,1));
$xml = new DomDocument;
$xpath = new DOMXpath($xml);
$dept = $xml->createElement("department");
$xml->appendChild($dept);
// loop and build the DOM
while($row = $result->fetch_assoc()){
$staff = $xml->createElement("staff");
// foreach($row as $col => $val) $staff->setAttribute($col, $val);
$staff->setAttribute("staff_id", $row["staff_id"]);
$staff->setAttribute("name", $row["name"]);
$staff->setAttribute("parent_staff_id", $row["parent_staff_id"]);
if(is_null($row["parent_staff_id"])){
$dept->setAttribute("dept_id", $row["dept_id"]);
$dept->setAttribute("department_name", $row["department_name"]);
$dept->appendChild($staff);
}
else{
$qry = sprintf("//*[#staff_id = '%d']", $row["parent_staff_id"]);
$parent = $xpath->query($qry)->item(0);
if(!is_null($parent)) $parent->appendChild($staff);
}
}
$result->close();
$conn->close();
echo $xml->saveXML();
?>
XML Output
<department dept_id="2" department_name="Mathematics">
<staff staff_id="1" name="f00" parent_staff_id="">
<staff staff_id="5" name="gamma" parent_staff_id="1"/>
<staff staff_id="6" name="delta" parent_staff_id="1">
<staff staff_id="7" name="zeta" parent_staff_id="6">
<staff staff_id="2" name="bar" parent_staff_id="7"/>
<staff staff_id="8" name="theta" parent_staff_id="7"/>
</staff>
</staff>
</staff>
</department>
SQL Stuff
-- TABLES
drop table if exists staff;
create table staff
(
staff_id smallint unsigned not null auto_increment primary key,
name varchar(255) not null
)
engine = innodb;
drop table if exists departments;
create table departments
(
dept_id tinyint unsigned not null auto_increment primary key,
name varchar(255) unique not null
)
engine = innodb;
drop table if exists department_staff;
create table department_staff
(
dept_id tinyint unsigned not null,
staff_id smallint unsigned not null,
parent_staff_id smallint unsigned null,
primary key (dept_id, staff_id),
key (staff_id),
key (parent_staff_id)
)
engine = innodb;
-- STORED PROCEDURES
drop procedure if exists department_hier;
delimiter #
create procedure department_hier
(
in p_dept_id tinyint unsigned,
in p_staff_id smallint unsigned
)
begin
declare v_done tinyint unsigned default 0;
declare v_dpth smallint unsigned default 0;
create temporary table hier(
dept_id tinyint unsigned,
parent_staff_id smallint unsigned,
staff_id smallint unsigned,
depth smallint unsigned
)engine = memory;
insert into hier select dept_id, parent_staff_id, staff_id, v_dpth from department_staff
where dept_id = p_dept_id and staff_id = p_staff_id;
/* http://dev.mysql.com/doc/refman/5.0/en/temporary-table-problems.html */
create temporary table tmp engine=memory select * from hier;
while not v_done do
if exists( select 1 from department_staff e
inner join hier on e.dept_id = hier.dept_id and e.parent_staff_id = hier.staff_id and hier.depth = v_dpth) then
insert into hier select e.dept_id, e.parent_staff_id, e.staff_id, v_dpth + 1 from department_staff e
inner join tmp on e.dept_id = tmp.dept_id and e.parent_staff_id = tmp.staff_id and tmp.depth = v_dpth;
set v_dpth = v_dpth + 1;
truncate table tmp;
insert into tmp select * from hier where depth = v_dpth;
else
set v_done = 1;
end if;
end while;
select
hier.dept_id,
d.name as department_name,
s.staff_id,
s.name,
p.staff_id as parent_staff_id,
p.name as parent_name,
hier.depth
from
hier
inner join departments d on hier.dept_id = d.dept_id
inner join staff s on hier.staff_id = s.staff_id
left outer join staff p on hier.parent_staff_id = p.staff_id;
drop temporary table if exists hier;
drop temporary table if exists tmp;
end #
delimiter ;
-- TEST DATA
insert into staff (name) values
('f00'),('bar'),('alpha'),('beta'),('gamma'),('delta'),('zeta'),('theta');
insert into departments (name) values
('Computing'),('Mathematics'),('English'),('Engineering'),('Law'),('Music');
insert into department_staff (dept_id, staff_id, parent_staff_id) values
(1,1,null),
(1,2,1),
(1,3,1),
(1,4,3),
(1,7,4),
(2,1,null),
(2,5,1),
(2,6,1),
(2,7,6),
(2,8,7),
(2,2,7);
-- TESTING (call this sproc from your php)
call department_hier(1,1);
call department_hier(2,1);