使用PHP / MySQL插入闭合表

Question

I'm trying to get my head around using a closure table in PHP/MySQL specifically Codeigniter, for an application I am building that has areas, and each area can have a number of locations, departments etc.

I have been using http://www.slideshare.net/billkarwin/models-for-hierarchical-data to learn how it works.

And I have also been using https://gist.github.com/dazld/2174233 to help me get to grips with accessing the data, which I can do fine, I've adapted the methods to pull out data as I need it, which is all good.

But now I'm trying to insert data, and I cannot get my head around it. So I've adapted the add method from the above script, but I don't understand it, and I cannot get it to work

Heres the area table

+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| area_id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| area_title | varchar(40) | NO   |     | NULL    |                |
| area_name  | varchar(40) | NO   |     | NULL    |                |
| org_id     | int(11)     | NO   |     | NULL    |                |
+------------+-------------+------+-----+---------+----------------+

Heres the area_hierarchy table

+------------+---------+------+-----+---------+----------------+
| Field      | Type    | Null | Key | Default | Extra          |
+------------+---------+------+-----+---------+----------------+
| id         | int(11) | NO   | PRI | NULL    | auto_increment |
| ancestor   | int(11) | NO   |     | NULL    |                |
| descendant | int(11) | NO   |     | NULL    |                |
| lvl        | int(11) | NO   |     | NULL    |                |
+------------+---------+------+-----+---------+----------------+

Heres the method I'm trying to use to add entries:

public function add($node_id, $target_id) {

    $sql = 'SELECT ancestor, '.$node_id.', lvl+1
            FROM area_hierarchy
            WHERE descendant = '.$target_id.'
            UNION 
            SELECT '.$node_id.','.$node_id.',0';

    $query = 'INSERT INTO area_hierarchy (ancestor, descendant,lvl) ('.$sql.')';

    $result = $this->db->query($query);

    return $result;

}

So specifically, what is $node_id, and $target_id.

What is the lvl+1?

So if I add a new top level area, what data do I pass to this method?

The query also fails and gives a syntax error after UNION

Answer 1

$node_id would be the value of the area_id column from the area table. Presumably, that's a row you just added to the area table.

$target_id would be the value of the area_id column from another row in the area table, the row you are identifying as the "parent", the direct ancestor.

The lvl+1 is the generation, or level in the hierarchy... how "far away" a descendant is from it's parent.

If you are adding a new "top level" area, you would use $node_id for $target_id .

One of the best ways to understand this is to look at the "current state" of the area_hierarchy table, and what the "end state" of the table should be when a node is added.

With just a top level node in the tree, area_id=11, the area_hierarchy table would look like this:

 ancestor descendant lvl 
 -------- ---------- ---
 11       11         0

If we add another row to the area table, area_id=22, as a child (direct descendant) of area_id=11, we need to add these two rows to area_hierarchy:

 ancestor descendant lvl 
 -------- ---------- ---
 11       22         1
 22       22         0

If we add another row to area table, area_id=333, as a child of area_id=22, we need to add these rows to the area_hierarchy table:

 ancestor descendant lvl 
 -------- ---------- ---
 11       333        2
 22       333        1
 333      333        0

Notice that the first two rows we need to add look a lot like the rows that already exist in the table, where descendant=22. The difference is that descendant on the new rows is 333 instead of 22, and value of lvl is one more than it was on the rows we already had in the table.

And that third row we need to add is the reference to itself. Just like we had area_id=11 as a "parent" of itself.

That third row we get from the part after the UNION. The first two rows we get by copying the rows already in area_hierarchy , replacing the value of the descendant column with the id of the node we're adding (333), and increasing lvl by 1.

Once you understand what rows need to be added, and how we can derive them by copying/modifying other rows in the table, then the SQL starts to make sense.

---

You would accomplish adding those three rows for area_id=333, calling the add function:

 add(333,22);

If you want to add area_id=4444 as a new "top level" in the hierarchy:

 add(4444,4444);