I’ve come across a lot of people having a problem when representing data from their databases; hierarchical data provides us with a particular problem when displaying the relationships and containers required.

An example of hierarchical data

As an example, let’s say you have forums within categories. The category “Web Programming” may have forums like “PHP”, “JavaScript” and so on, so we want to list each forum under each category. The problem here is that you’ll have:

• a table for categories
• one for forums (with a foreign key for categories)
• one for posts, with a foreign key for forums
• and probably one for comments.

This fits quite neatly into a hierarchy: each comment belongs to a post, which belongs to a forum, which belongs to a category.

Using multiple resultsets

If you want to display the Categories with Forums under them in your page, one way to do this is by setting up multiple resultsets, each focusing on one level of the hierarchy. To achieve this, you’d need to get a resultset for the categories first, and then iterate over this list with another loop for your forum list.

The following code is an example of iterating using nested queries:

$cat_rs = mysql_query("select id, name from categories");
while($cat_row = mysql_fetch_array($cat_rs)){
     // print category name from $cat_row[1]
     $forum_rs = mysql_query("select name... "
                          . "from forums "
                          . "where cat_id = '" . $cat_row[0] ."'");
     while($forum_row = mysql_fetch_array($forum_rs)){
        //print forum stuff
     }
}

Although this matches the hierarchical nature of the data, and is pretty straightforward and intuitive to understand, it is somewhat inefficient; it issues a select statement for each category, plus one to list the categories in the first place. If there are many categories, the page might take some time to load as all queries are issued and processed.
Using a SQL JOIN: a single resultset
Another way to solve the problem would be to retrieve all relevant data in a single resultset, and use PHP to iterate over the rows, deciding on when to print each individual section.
A join will give you a single recordset with the category in one column, and the forum in another, giving you many rows for a single category.
$join_rs = mysql_query("select c.id, c.name, f.name,... "
                        . "from categories c inner join forums f "
                        . "on c.id = f.cat_id "
                        . "order by c.id"); // this line is crucial
$current_category = "";
while($join_row = mysql_fetch_array($join_rs)){
	if($join_row[0] != $current_category){
                //set up new category headings here
                $current_category = $join_row[0];
        }
        //print the forum stuff in the current category
}

The PHP is a bit more complex, because you have to close divs or tables correctly, within the loop. The order by line in the above example is vital, because without it, the categories may be dotted throughout the resultset. As we want all forum data related to a single category to appear together, we want rows belonging to that category to be adjacent in the resultset. The order by clause achieves this.
Despite the complexity, there are still benefits to this approach. The earlier example was simple and intuitive, using nested queries for each parent element. On the other hand, in this example we’ve only issued one select statement, so the page is likely to load more quickly, given that the number of round trips to MySQL will be substantially fewer.