Solved

How to construct a treeview from a relational database

Posted on 1999-01-22
8
180 Views
Last Modified: 2010-04-06
A table with fields son,father,data.
son is the son of the father in one tree.
now,please tell me any method to get the treeview from the
table.,
Thank you.
0
Comment
Question by:mathematics
  • 3
  • 2
  • 2
  • +1
8 Comments
 
LVL 27

Expert Comment

by:kretzschmar
ID: 1362917
Hi Mathematics,

See my comment, at your q:

http://www.experts-exchange.com/topics/comp/lang/delphi/Q.10116479

meikl
0
 
LVL 1

Accepted Solution

by:
wamoz earned 50 total points
ID: 1362918
What you are describing is a classical problem known as the "Explosion of Parts" problem. It is intractable in a declarative language that does not support recursive evaluation, and unfortunately SQL falls into this category.

On the other hand the problem yields easily to iterative treatment.

What you should do is write a parametric SQL statement that selects all the offspring of a nominated individual, and run this each time a childless node is expanded. In the event handler procedure for node expansion you need to obtain the UID of the entity that the node represents, and use this as the parameter governing the list of entities that your query will return. You use this result set to add children to the node that is expanding.

This approach can even deal with circular references.
0
 
LVL 27

Expert Comment

by:kretzschmar
ID: 1362919
hi mathematics,

I've pasted my comment here

procedure TForm1.Button1Click(Sender: TObject);
var
  Found : Boolean;
  I : Integer;
  t : TTreeNode;
begin
  { Clear Treeview }
  while treeview1.Items.count > 0 do treeview1.Items[0].delete;
  { Create a root }
  treeview1.Items.Add(treeview1.TopItem,Table1.TableName);
  table1.open;
  table1.first;
  while not(Table1.eof) do
  begin
    i := 0;
    Found := False;

    { Search for Father }

    while Not(Found) and (i < TreeView1.Items.Count) do
    begin
      Found := Table1.FieldByName('Father').AsString = Treeview1.Items[i].Text;
      if Not(Found) then inc(i);
    end;

    If Found then  {Father exists, add son }
    begin
      t := TreeView1.Items.AddChild(TreeView1.Items[i],Table1.FieldByName('Son').AsString);
       { Do Something else with t, i.e. some data in the node.data option }
    end
    else
    begin     {Father not exists, add Father to root, add son }
      t := TreeView1.Items.AddChild(TreeView1.TopItem,Table1.FieldByName('Father').AsString);
       { Do Something else with t  i.e. some data in the node.data option}
      t := TreeView1.Items.AddChild(t,Table1.FieldByName('Son').AsString);
       { Do Something else with t  i.e. some data in the node.data option}
    end;
    Table1.Next; {Next Record}
  end;
  table1.Close;
end;


meikl
0
Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 

Author Comment

by:mathematics
ID: 1362920
Thanks everybody.
0
 
LVL 27

Expert Comment

by:kretzschmar
ID: 1362921
Hi mathematics,

i wonder that the theoretical answer from wamoz helped you?

meikl

0
 

Author Comment

by:mathematics
ID: 1362922
Thank kretzschmar!
Your comment is useful for me before.
I am glad to every answer.
Hope you understand.
0
 

Expert Comment

by:MBerlin
ID: 2048901
wamoz theoretical answer was useful to me too.
I've come across a similar problem and solved it as wamoz mentioned but I wondered if SQL had a better way that I did not know.
wamoz says no, sql does not have a better way.
thanks for putting that doubt to rest
0
 
LVL 1

Expert Comment

by:wamoz
ID: 2050011
The failure of SQL to support recursive evaluation is by design (ANSI committee). Relations containing circular references CANNOT be automatically materialised because they contain an infinite number of tuples.

I have written custom SQL interpreters that deal with this problem by terminating branch evaluation on detection of repeated tuples. However, when presenting this kind of information in a tree widget, it is actually *easier* to fetch one "child set" at a time as previously described, and as this leads to optimal performance, this is the preferred approach.

As a famously anonymous person once said, "If it aint broke, don't fix it."
0

Featured Post

Free Tool: Site Down Detector

Helpful to verify reports of your own downtime, or to double check a downed website you are trying to access.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

A lot of questions regard threads in Delphi.   One of the more specific questions is how to show progress of the thread.   Updating a progressbar from inside a thread is a mistake. A solution to this would be to send a synchronized message to the…
Have you ever had your Delphi form/application just hanging while waiting for data to load? This is the article to read if you want to learn some things about adding threads for data loading in the background. First, I'll setup a general applica…
In an interesting question (https://www.experts-exchange.com/questions/29008360/) here at Experts Exchange, a member asked how to split a single image into multiple images. The primary usage for this is to place many photographs on a flatbed scanner…

808 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question