How do I extract and display hierarchical data from my database? - php

Question

I have two tables.
The chapters table has the columns id and name.
The chapters_chapter table has columns id, master_id, and slave_id.
Lets say that the chapters table has 7 records:
id name
1 test01
2 test02
3 test03
4 test04
5 test05
6 test06
7 test07
And in the chapters_chapters table I have these records:
id master_id slave_id
1 1 5
2 1 6
3 6 7
4 7 2
Given that data, how can I extract the hierarchy of that data so that it looks like this?
test01
test05
test06
test07
test02
test03
test04

Answer 1

So this was kind of a pain because of the fact that we had to have the hierarchy stored in the DB. Because of this, each item can have multiple children, and each child can have multiple parents.
This second part means we cannot simply loop through the list once and be done with it. We might have to insert an item in multiple places in the hierarchy. While you probably won't actually structure your data that way, the database schema you've described supports this scenario, so the code must support it too.
Here's a high-level version of the algorithm:
Query both tables
Create a map (array) of a parent (number) to its children (another array)
Create a set of items that are children (array of numbers)
Create a function that displays a single item, indenting it to the desired depth.
If that item has children, this function increases the depth by one, and calls itself recursively
Loop through all items that aren't children (root items).
Call the function for each of those items, with a desired depth of 0 (no indent).
Here's two hours work. Enjoy :)
Note that I stuck it within a <pre> block, so you might have to mess with how the indentation is done (output something other than two spaces, mess with the style of the divs, etc).
<?php
$con = mysql_connect("localhost", "test_user", "your_password");
if(!$con)
{
die("could not connect to DB: " . mysql_error());
}
mysql_select_db("your_db", $con);
// get chapters
$chapters = array();
$result = mysql_query("SELECT * FROM chapters");
while($row = mysql_fetch_array($result))
{
$id = $row["id"];
$name = $row["name"];
$chapters[$id] = $name;
}
// get chapters_chapters - We'll call it "parent/child" instead of "master/slave"
$parent_child_map = array();
$is_child = array();
$result = mysql_query("SELECT master_id, slave_id FROM chapters_chapters");
while($row = mysql_fetch_array($result))
{
$parent_id = $row["master_id"];
$child_id = $row["slave_id"];
$children = $parent_child_map[$parent_id];
if($children == null)
{
$children = array();
}
$children[] = $child_id;
$parent_child_map[$parent_id] = $children;
$is_child[$child_id] = true;
}
// display item hierarchically
$display_item_and_children = function($id, $name, $depth)
use ($chapters, $parent_child_map, &$display_item_and_children)
{
echo "<div><pre>";
// indent up to depth
for($i = 0; $i < $depth; $i++)
{
echo " ";
}
echo "id: " . $id
. " name: " . $name
. "</pre></div>";
// if there are children, display them recursively
$children = $parent_child_map[$id];
if($children != null)
{
foreach($children as $child_id)
{
$child_name = $chapters[$child_id];
$display_item_and_children($child_id, $child_name, $depth + 1);
}
}
};
// display all top-level items hierarchically
foreach($chapters as $id => $name)
{
// if it is a top-level item, display it
if($is_child[$id] != true)
{
$display_item_and_children($id, $name, 0);
}
}
mysql_close($con);
?>
And here's a screenshot:

Answer 2

The question becomes how complex you want your solution to be. I'd do it with the following pseudo code.
SELECT all the chapters
SELECT all the *chapters_chapters*
loop over the chapters to create an array chapter objects
loop over the `chapters_chapters* and create the relationships using the chapter objects
Essentially you're creating a link-list.