Lesson 13 - The Columns of a Table

The Columns of a Table


Introduction



A column is a technique of categorizing some values that belong to a table. Based on this, one category from a list of a video application can be made of video titles. Another category can contain the years the videos were released. Yet another category can contain a number used to identify the format of the video. Here is an example of a table:

Video Title Director © Year Length Format Rating
A Few Good Men Rob Reiner 1992 138 Minutes 1 R
The Distinguished Gentleman Jonathan Lynn 112 Minutes 2 R
The Lady Killers Joel Coen & Ethan Coen 104 Minutes 2 R
Fatal Attraction Adrian Lyne 1987 120 Minutes 1 R
Her Alibi Bruce Beresford 1989 94 Minutes 2 PG-13
The Manchurian Candidate Jonathan Demme 2004 129 Minutes 2 R

A category of information of a table is called a column. The string on top of each column allows the user to identify what that column is used for. That string is called the column header.

To support the columns of a table, the .NET Framework provides the DataColumn class.

Practical Learning: Introducing Columns



Start Microsoft Visual C# and create a Console Application named VideoCollection2
To create a new class, in the Class View, right-click VideoCollection2 -> Add -> Class...
Set the Name to Video and press Enter
Change the file as follows:
using System;
using System.Data;

namespace VideoCollection2
{
public class Video
{
private DataSet dsVideoCollection;

private DataTable tblActors;
private DataTable tblVideos;

public Video()
{
dsVideoCollection = new DataSet("VideoCollection");

tblActors = new DataTable("Actors");
dsVideoCollection.Tables.Add(tblActors);

tblVideos = new DataTable("Videos");
dsVideoCollection.Tables.Add(tblVideos);
}

public void ShowTables()
{
int i = 1;

Console.WriteLine("Video Collection - Tables");
foreach (DataTable tbl in dsVideoCollection.Tables)
Console.WriteLine("{0}. {1}", i++, tbl.TableName);
}
}
}


Access the Program.cs file and change it as follows:
using System;

namespace VideoCollection2
{
public class Program
{
static int Main(string[] args)
{
Video vdo = new Video();
vdo.ShowTables();

return 0;
}
}
}


To save the file, on the Standard toolbar, click the Save All button
Accepts all defaults and click Save
Execute the application to make sure it is working fine:
Video Collection - Tables
1. Actors
2. Videos
Press any key to continue . . .


Close the DOS window
Creating a Column



To create a column, you can first declare a variable of type DataColumn. The DataColumn class is equipped with five constructors. The default constructor allows you to create a column without giving details. Here is an example:

using System;
using System.Data;

public class VideoCollection
{
public VideoCollection()
{
DataColumn colCategoryID = new DataColumn();
}
}
If you are planning to reference the column from more than one method, you should declare it in a class. Here is an example:

using System;
using System.Data;

public class VideoCollection
{
private DataColumn colCategoryID;

public VideoCollection()
{
colCategoryID = new DataColumn();
}
}
To distinguish them, each column must have a specific and unique name. The name of a column allows you and the compiler to identify a particular column. The name must follow the rules of variables in C#. To specify the object name of a column, when creating it, you can use the second constructor whose syntax is:

public DataColumn(string name);
This constructor expects as argument the name of the column. Here is an example:

using System;
using System.Data;

public class VideoCollection
{
private DataColumn colCategoryID;

public VideoCollection()
{
colCategoryID = new DataColumn("CategoryID");
}
}
The name of a column is supported by the ColumnName property of the DataColumn class. This property is of type string. If you have already declared a DataColumn object, to specify or change its name, assign the desired string to the DataColumn.ColumnName property. Here is an example:

using System;
using System.Data;

public class VideoCollection
{
private DataColumn colCategoryID;
private DataColumn colCategory;

public VideoCollection()
{
colCategoryID = new DataColumn("CategoryID");
colCategory = new DataColumn();
colCategory.ColumnName = "Category";
}
}
Based on these descriptions, the minimum information needed to create a column is a name. If you don't specify a name, a default name is assigned to the new column.