Building a Mysql table-tree using query instead of loops in PHP - php

Question

I have a category tree that contains up to 3 levels of children-categories, like this:
houseproducts->livingroom->sofas->twoseats
houseproducts->livingroom->sofas->threeseats
houseproducts->livingroom->sofas->fourseats
So for each sublevel, I do a SELECT based on the mothers category-id. This is done in a PHP-loop like the code below, but I guess it could be done in one single Mysql-query, for better performance. I have tried different JOINS but find it really difficualt. Any advice will be highly appreciated.
function build_category_tree()
{
$cat = array();
// main category loop
$r1 = mysql_query("SELECT cat_id,cat_name FROM categories WHERE cat_mother=0 OR cat_mother='' ORDER BY cat_name");
while ($row=mysql_fetch_assoc($r1))
{
$cat[$row['cat_id']] = $row['cat_name'];
// check for subcategories
$r2 = mysql_query("SELECT cat_id,cat_name FROM categories WHERE cat_mother='".$row['cat_id']."'");
while ($subrow=mysql_fetch_assoc($r2))
{
$cat[$subrow['cat_id']] = ' - '.$subrow['cat_name'];
// check if there is subcats for the current subcategory
$r3 = mysql_query("SELECT cat_id,cat_name FROM categories WHERE cat_mother='".$subrow['cat_id']."'");
while ($subrow2=mysql_fetch_assoc($r3))
{
$cat[$subrow2['cat_id']] = ' -- '.$subrow2['cat_name'];
// check if there is subcats for the current subcategory
$r4 = mysql_query("SELECT cat_id,cat_name FROM categories WHERE cat_mother='".$subrow2['cat_id']."'");
while ($subrow3=mysql_fetch_assoc($r4))
{
$cat[$subrow3['cat_id']] = ' --- '.$subrow3['cat_name'];
}
}
}
}
return $cat;
}

Answer 1

I would read your entire table into an array and segment that array by the mother's keys

Answer 2

Try this:
SELECT l1.cat_id AS l1_cat_id
,l1.cat_name AS l1_cat_name
,l2.cat_id AS l2_cat_id
,l2.cat_name AS l2_cat_name
,l3.cat_id AS l3_cat_id
,l3.cat_name AS l3_cat_name
,l4.cat_id AS l4_cat_id
,l4.cat_name AS l4_cat_name
FROM categories AS l1
JOIN categories AS l2
ON l2.cat_mother = l1.cat_id
JOIN categories AS l3
ON l3.cat_mother = l2.cat_id
JOIN categories AS l4
ON l4.cat_mother = l3.cat_id
WHERE l1.cat_mother=0 OR l1.cat_mother=''
ORDER BY l1_cat_name, l2_cat_name, l3_cat_name, l4_cat_name