Convert a series of parent-child relationships into a hierarchical tree?

Question

I have a bunch of name-parentname pairs, that I\'d like to turn into as few heirarchical tree structures as possible. So for example, these could be the pairings:

Answer 1

Old question, but I too had to do this and the examples with recursion gave me a headache. In my database we have a locations table, which was a loca_id PK (Child) and self referencing loca_parent_id (Parent).

The aim is to represent this structure in HTML. The simple query can of couyrse return the data is some fixed order but I found not well enough to display such data in a natural way. What I really wanted was Oracle tree walk handling with LEVEL to help with display.

I decided to use the idea of a 'path' to uniquely identify each entry. For example:

Sorting the array by the path should make it easier to process for meaningful display.

I realise that use of associative arrays and sorts is cheating as it hides the recursive complexity of the operations, but to me this look simpler:

prepare($sql);
    $stmt->execute();
    $result = $stmt->get_result();
    $data = $result->fetch_all(MYSQLI_ASSOC);
    
    $printed = array();
    
    // To get tree hierarchy usually means recursion of data.
    // Here we will try to use an associate array and set a
    // 'path' value to represent the hierarchy tree in one
    // pass. Sorting this array by the path value should give
    // a nice tree order and reference.
// The array key will be the unique id (loca_id) for each row.
// The value for each key will the complete row from the database.
// The row contains a element 'loca_path' - we will write the path
// for each row here. A child's path will be parent_path/child_name.
// For any child we encounter with a parent we look up the parents path
// using the loca_parent_id as the key.
// Caveat, although tested quickly, just make sure that all parents are
// returned first by the query.
    
    foreach ($data as $row)
    {
    
       if ( $row['loca_parent_id'] == '' ) // Root Parent
       {
          $row['loca_path'] = $row['loca_name'] . '/';
          $printed[$row['loca_id']] = $row;
       }
       else // Child/Sub-Parent
       {
          $row['loca_path'] = $printed[$row['loca_parent_id']]['loca_path'] . $row['loca_name'] . '/';
          $printed[$row['loca_id']] = $row;
       }
    }
    
    // Array with paths built, now sort then print
    
    array_multisort(array_column($printed, 'loca_path'), SORT_ASC, $printed);
    
    foreach ( $printed as $prow )
    {
       print_row ( $prow );
    }
    ?>