Create a tree from a resultset

[Mike Boota]

Hi,

I need to create a tree structure from a resultset and show them into my JSP. Here is my table sql (grup).

CREATE TABLE GRUP (
ID_GRUP int(11) NOT NULL default '0',
ID_GRUP_PARENT int(11) NOT NULL default '0',
ORDER_BY int(11) NOT NULL default '1',
DESCRP varchar(100) default NULL,
PRIMARY KEY ('ID_GRUP'),
)

What I really need is a recursive method to search for all the children of each row and build a tree heirarchy. e.g. the data may look like

ID_GRUP   ID_GRUP_PARENT      ORDER_BY    DESCRP
1               0                 1        It itself is the parent
2               1                 1        It's Parent is ID_GRUP=1
3               0                 2        It itself is the parent
4               2                 1        It's parent is ID_GRUP=2
5               1                 2        It's Parent is ID_GRUP=1
6               4                 1        It's Parent is ID_GRUP=4
7               2                 2        It's parent is ID_GRUP=2
8               2                 3        It's parent is ID_GRUP=2

So looking at the top data my final tree structure should look like

1
  2
    4
      6
    7
    8
  5
3

and so on. ORDER_BY show the order in which each element should display in the heirarchy. I am really struggling to find a way on how to have the heirarchy built and really need some help.

Thanks


Here is the SQL i have to het the data

SELECT ID_GRUP, ID_GRUP_PARENT, DESCRP FROM GRUP ORDER BY ORDER_BY.

[Rob Spoor]

I usually use a Map for this. Something like this (pseudo code):

// first create all
Map<ID,Node> nodes = new LinkedHashMap<ID,Node>(); // to keep insertion order
for each record
{
    Node node = create node from record
    nodes.put(node.id, node);
}
// now link all
List<Node> roots = new ArrayList<Node>();
for (Node node: nodes.values())
{
    Node parent = nodes.get(node.parentID);
    if (parent != null)
    {
        node.parent = parent;
    }
    else
    {
        roots.add(node);
    }
}

When done roots contains all root nodes. If you have one single tree it should contain only one element.