Question

Consider the database table: folders

folders
id   parent_id    name
1    0            a  
2    1            b  
3    2            c

‘b’ is a folder inside ‘a’ and thus it’s parent_id is the id of ‘a’.
Folders with a parent_id of 0 simply means it’s in the root folder.

I wrote a recursive function in php that helps me get the ID of a path that I’m interested in.

For example:

echo get_folder_id('a/b/c'); // 3 (3 SQL queries)  
echo get_folder_id('a'); // 1 (1 SQL query)  
echo get_folder_id('a/b'); // 2 (2 SQL queries)  
echo get_folder_id('a/c'); // false (2 SQL queries)

Problem: For each folder in the path, I had to make a SQL query to the DB server.

Question: Is there a way I can reduce the number of queries if the path is ‘a/b/c’?

Here’s my current working solution for reference.

function get_folder_id($path, $parent_id=0) {
    $path = explode('/', $path);
    $id = 0;

    //if there's only 1 folder in the path, query the database for the ID
    if (count($path) == 1) {
        $rs = $this->db->select('id')
                ->from('folders')
                ->where('name', $path[0])
                ->where('parent_id', $parent_id)
                ->limit(1)
                ->get();

        if ($rs->num_rows() == 0) return FALSE;

        $result = $rs->row_array();
        return $result['id'];
    }

    foreach($path AS $i=>$p) {
        if ($i==0 && $p=='') continue; //if a path starts with / move on to the next item
        $id = $this->get_folder_id($p, $id);
    }

    return $id;
}

Note: I’m using CodeIgniter framework but this isn’t related to my question. Just for folks wondering why I used $this->get_folder_id() and $this->db

Answer 1

One solution I just came up is to use a series of LEFT JOINs on the same table.

SELECT tb3.id FROM folders AS tb1
LEFT JOIN folders AS tb2 ON tb2.parent_id=tb1.id
LEFT JOIN folders AS tb3 ON tb3.parent_id=tb2.id
WHERE tb1.name='a' AND tb2.name='b' AND tb3.name='c'
LIMIT 1

Based on the number of folders in the path, I can dynamically generate the above SQL.
I’m not sure if this is the perfect solution, but right now it’s working as intended.

Any feedback on whether this is a good idea or not is greatly appreciated!

$path = 'a/b/c';
$path = explode('/', $path);

$sql = "SELECT tb".count($path).".id FROM folders AS tb1";
for($x=1, $max = count($path); $x<=$max; $x++) {
    $sql .= " LEFT JOIN folders AS tb".($i+2);
    $sql .= " ON tb".($i+2).".parent_id = tb"($i+1).".id";
}

$sql .= " WHERE 1";

foreach($path AS $x => $p) {
    $sql .= " AND tb".($x=1).".name='".$p."'";
}

$sql .= " LIMIT 1";