How to fetch query where sub/child create new UL

Question

I am trying to make un-order list for parent child categories where if there is any child category than it will create another un-order list ( like indented text) so user can understand properly.

I have fetch sql but with foreach I don't understand how to set so where child category only will display under parent category by creating another un-order list under the parent category.

Here is my code

$query_cat =    "SELECT * FROM ^categories";
$query = qa_db_query_sub($query_cat);
$catsid = qa_db_read_all_assoc($query);

echo '<UL>';
foreach ($catsid as $catid){
    echo '<LI>'. $catid['title'].' '. $catid['categoryid'].'</LI>';
}
echo '</UL>';

So final result would be

• First Category
  • Sub Category1
• Second Category

EDIT:

After modified code with @vlcekmi3 answer https://stackoverflow.com/a/13451136/1053190 I am getting this result

Now how to exclude subcategory from parent list?

Answer 1

you can use complicated query or something like this

foreach ($catsid as $catid) {
    ...
    $subquery_cat = "SELECT * FROM ^categories WHERE parentid='".$catid['categoryid']."'";
    $query = qa_db_query_sub($subquery_cat);
    $subcatsid = qa_db_read_all_assoc($query);
    // wrap into html
    ...
}

Answer 2

There's no really easy solution for this with your design. The most effective way would be to add column like order_in_list (and maybe depth_in_list).

They would be pre calculated in loop (pseudocode):

START TRANSACTION
UPDATE t1 SET order_in_list = 0 // Restart whole loop

$ids = array(0);

while $id = array_shift($ids){
   $record = SELECT * FROM t1 WHERE id = $id // Get id details, order_in_list is important
   $children = SELECT * FROM t1 WHERE parent_id = $id // get list of all childs
   // If it's root element, start indexing from 0
   $root_order = ($record ? $record->order_in_list : 1) 
   $child_no = count($children) // How many child will be adding

   // No children, nothing to do:
   if $child_no < 1{
      continue;
   }

   append_to_array($ids, $children) // Store ids to process

   // Shift all later records, we'll be creating gap in order_in_list 1,2,3,4,5
   // To 1,2,5,6,7 to insert items on places 3,4
   UPDATE t1 SET order_in_list = (order_in_list + $child_no)
      WHERE order_in_list > $record->order_in_list

   // Okay, set IDs for direct children
   foreach( $children as $child){
       UPDATE t1 SET order_in_list = $root_order, depth_in_list = $record->depth_in_list+1
          WHERE id = $child->id
       $root_order++;
   }
}
COMMIT

This way you'll get records like:

First category, 1, 1
Second category 3, 1
Sub category, 2, 2

Which you could display with simple loop:

$last_depth = 0;
foreach( (SELECT * FROM t1 ORDER by `order_in_list`) as $row){
    if( $last_detph > $row['depth_in_list'])){
       // Close level </ul>
    } else if($last_detph < $row['depth_in_list']){
       // Opening level <ul>
    } else {
       // The same depth
    }
    $last_depth = $row['depth_in_list'];
}

---

Without modifying database

It would be probably most effective to build two arrays containing root elements and all elements:

$root_elements = array();
$all_elements = array();

foreach( (SELECT * FROM t1) as $row){
    // Store details into all_elements, note that entry may have already be created when
    // processing child node
    if( isset( $all_elements[$row['id']])){
      // set details
    } else {
      $all_elements[$row['id']] = $row;
      $all_elements[$row['id']]['children'] = array(); // Array of child elements
    }

    if( $row['parent_id'] == NULL){
        $all_elements[] = $row['id'];  // Add row element
    } else {
        if( isset( $all_elements[ $row[ 'parent_id']])){
            $all_elements[ $row[ 'parent_id']]['children'][] = $row['id'];
        } else {
            // Create new record:
            $all_elements[ $row[ 'parent_id']] = array();
            $all_elements[ $row[ 'parent_id']]['children'] = array($row['id']);
        }
    }
}

And then write it as:

foreach( $root_elements as $element_id){
    write_recursive( $all_elements[ $element_id]);
}

// And display
function write_recursive( $element)
{
   echo '<ul>...';
   if( count( $element['children'])){
      foreach( $element['children'] as $child){
         write_recursive( $all_elements[ $child]);
      }
   }
   echo '</ul>';
}

You better create class for that (to replace using global variables), but you should have a solid way to do this. Anyway try avoid using this with large number of records (I wouldn't go past 2000-5000 menu entries), try to at least cache it.

Note: solutions are oriented towards minimal number of requests on database when displaying list.