Count all child nodes of hierarchical data in a table

Question

I want to count number of all child nodes under any level of tree structure maintained in a table using adjacency model (parent-child key). Table structure and data looks l

Answer 1

Below is a PHP based solution:

function countChildren($startId) {
    $directDescendents = *_query("SELECT id FROM Table WHERE parentid = ?", array( $startId ));
    $count = *_num_rows($directDescendents);
    while($row = *_fetch_array($directDescendents))
        $count += countChildren($row['id']);
    return $count;
}

$numChildren = countChildren(2); // Number of Children for 'B'

Replace *_num_rows and *_fetch_array with whatever functions for the SQL extension you are using. This won't be as efficient as a pure SQL solution, but it will work. The way I'm querying in the function is assuming bound parameters, but execute the query as you like.