Marten’s blog

Tree traversal is a systematic way of ordering the nodes of a tree. When you are going throw three traversal structure, you will visit each node exactly once.

I use this method in work a few years ago and using it till now. Imagine that you have category table (i’ll be using MSSQL syntax for this post):

CREATE TABLE Categories
(
  ID int IDENTITY(1,1) NOT NULL,
  CategoryName NVARCHAR(100) NOT NULL,
  ParentID INT NULL,
  CONSTRAINT pk_CategoryID PRIMARY KEY (ID)
)

INSERT INTO Categories (CategoryName, ParentID) VALUES ('Computers', NULL); -- ID: 1
INSERT INTO Categories (CategoryName, ParentID) VALUES ('PC', 1); -- ID: 2
INSERT INTO Categories (CategoryName, ParentID) VALUES ('AMD', 2); -- ID: 3
INSERT INTO Categories (CategoryName, ParentID) VALUES ('Pentium', 2); -- ID: 4
INSERT INTO Categories (CategoryName, ParentID) VALUES ('Notebooks', 1); -- ID: 5
INSERT INTO Categories (CategoryName, ParentID) VALUES ('Books', NULL); -- ID: 6
INSERT INTO Categories (CategoryName, ParentID) VALUES ('Sci-fi', 6); -- ID: 7
INSERT INTO Categories (CategoryName, ParentID) VALUES ('Fantasy', 7); -- ID: 8

We have two parent categories ‘Computers’ and ‘Books’. Both of them have subcategories and ‘Computers’ category has subsubcategories as well. Now you will need to get all products from ‘Computers’ category and all subcategories. With this database structure it will be realy hard to get all products. You will need to get all category Ids and join them in IN clause. There is better way how to get them, it’s called tree traversal.

Tree traversal example

Here is image which shows previous data we inserted into category table.

Now we will add numbers to left and right side of each elipse, begining from left parent category, go to subcategories and continuing to right side.

Numbers on left side will be called ‘lft, on right side ‘rtg’. We can now update database structure to contain these informations.

CREATE TABLE Categories
(
  ID INT NOT NULL,
  CategoryName NVARCHAR(100) NOT NULL,
  ParentID INT NULL,
  lft INT NOT NULL,
  rtg INT NOT NULL,
  CONSTRAINT pk_CategoryID PRIMARY KEY (ID)
)

INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Computers', NULL, 1, 10); -- ID: 1
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('PC', 1, 2, 7); -- ID: 2
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('AMD', 2, 3, 4); -- ID: 3
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Pentium', 2, 5, 6); -- ID: 4
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Notebooks', 1, 8, 9); -- ID: 5
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Books', NULL, 11, 16); -- ID: 6
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Sci-fi', 6, 12, 13); -- ID: 7
INSERT INTO Categories (CategoryName, ParentID, lft, rtg) VALUES ('Fantasy', 7, 14, 15); -- ID: 8

Get child categories

Now we can get all child categories of ‘Computers’ category.

SELECT * FROM Categories WHERE lft >= 1 AND rtg <= 10;

Get all parent categories

Another posibility is when we need to create breadcrumbs for actual product page. We know, that the product is in ‘Pentium’ category. To get all parent categories we will use similar sql to the one above

SELECT * FROM Categories WHERE lft <= 5 AND rtg >= 6 ORDER BY lft ASC;

Have the category any childs?

If you have the category data and need to know if the category have any child categories you will need to make another select which will search for all categories which have ParentID same as your category. With this you can only compare ‘lft’ and ‘rtg’. If ‘lft’ +1 = ‘rtg’ than the category doesn’t have any subcategories.

Database Database, MSSQL, MSSQL 2005, MSSQL 2008, SQL

Sometimes, maybe more the less, have developers really hard times. When it comes to some extra times when the deadline is really close. More than one year ago I saw video about Free hugs campaign, the idea of Juan Mann. I don’t even think about that this can be changed to some free hugs day for developers :-) We try to send this in our office to the rest of the team (especially to the ladies marketing team) and it’s successful.

Others

When I started developing web controls which are created by CreateChildControls method i run into problem that this control is surrounded by span tag.

[ToolboxData("<{0}:ExtendedLabel runat=\"server\" Text=\"Label\" />")]
public class ExtendedLabel : WebControl, INamingContainer
{
  protected override void CreateChildControls()
  {
    Label label = new Label();
    label.Text = this.Text;
    this.Controls.Add (label);
  }
}

This example will product not so nice HTML code:

<span id=”ExtendedLabel”>
  <span name=”ExtendedLabel:Label”>Test string</span>
</span>

The first label come from RenderBeginTag method which contains by default span tag. What you can do is to override both RenderBeginTag and RenderEndTag.

protected override void RenderBeginTag(HtmlTextWriter writer)
{
  writer.Text = writer.Text + "<div id=\"myTag\">";
}
protected override void RenderBeginTag(HtmlTextWriter writer)
{
  writer.Text = writer.Text + "</div>";
}

Or remove this tag completely by overriding Render method.

Original:

protected override void Render(HtmlTextWriter writer)
{
  this.RenderBeginTag(writer);
  this.RenderContents(writer);
  this.RenderEndTag(writer);
}

New:

protected override void Render(HtmlTextWriter writer)
{
  this.RenderContents(writer);
}

In these examples you have full control over behavior of the control. These is another possibility how to change or remove the outlining tag. You can extend your web control from another class.

protected ExtendedLabel () : base()
{ }

.NET, C# .NET, C#, Web development, WebControl