Databases Fundamentals: Data Columns

We have defined a column as a technique 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:

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. The columns of a table are stored in a property called Columns of the DataTable class. The Columns property is an object of type DataColumnCollection. The DataColumnCollection class provides everything that is necessary to create and manage any column. The DataColumnCollection class is in fact a collection of objects.

1. Start Notepad and, from what we learned in the lesson on tables, type the following:
    

   Imports System Imports System.Xml Imports System.data Class Exercise Private Shared dsVideoCollection As DataSet Private Shared tblVideoCategories As DataTable Private Shared tblDirectors As DataTable Private Shared tblRatings As DataTable Private Shared tblActors As DataTable Private Shared tblFormats As DataTable Public Shared Sub Main() dsVideoCollection = New DataSet("Videos") tblDirectors = new DataTable() tblDirectors.TableName = "Directors" dsVideoCollection.Tables.Add(tblDirectors) tblVideoCategories = new DataTable("Categories") dsVideoCollection.Tables.Add(tblVideoCategories) tblRatings = dsVideoCollection.Tables.Add() tblActors = dsVideoCollection.Tables.Add("Actors") tblFormats = dsVideoCollection.Tables.Add("Formats") End Sub End Class

2. To save the file, on the main menu, click File -> Save
3. Locate your CSharp Lessons folder and display it in the Save In combo box
4. Click the Create New Folder button. Type VideoCollection2 and press Enter twice to display the new folder in the Save In combo box
5. Change the Save As Type to All Files. Change the file name to Exercise.vb and click Save

A column of a table is an object of type DataColumn. Therefore, 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 more details. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Sub Main()

        Dim colCategoryID As DataColumn = New DataColumn

    End Sub

End Module

If you are planning to reference a column from more than one method or event, you should declare it globally, in the class of the form. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Imports System.Data

Module Exercise

    Private colCategoryID As DataColumn

    Sub Main()

        colCategoryID = New DataColumn

    End Sub

End Module

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 VBasic. To specify the object name of a column, when creating it, you can use the second constructor whose syntax is:

Public Sub New(ByVal columnName As String)

This constructor expects as argument the name of the column. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Imports System.Data

Module Exercise

    Private colCategoryID As DataColumn

    Sub Main()

        colCategoryID = New DataColumn("CategoryID")

    End Sub

End Module

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:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private colCategoryID As DataColumn
    Private colCategory As DataColumn

    Sub Main()

        colCategoryID = New DataColumn("CategoryID")
        colCategory = New DataColumn
        colCategory.ColumnName = "Category"
    End Sub

End Module

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.

To make a column part of a table, you must add it to the table's collection of columns. The columns of a table are grouped into the DataTable.Columns property, which is based on the DataColumnCollection class. This class is equipped with an Add() method that allows you to add a column to the table.

The DataColumnCollection.Add() method is overloaded with 3 versions. The first version uses the following syntax:

Overloads Public Overridable Function Add() As DataColumn

When called, this would create a new column and return it. The compiler would assign a default name to the column. If this is the first column, it would be named Column1. If it is the second column, it would be named Column2, and so on. You can still specify or change the name of a column created with the above version. To do this, assign the desired string to the DataColumn.ColumnName. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dtDirectors As DataTable
    Private colDirectorID As DataColumn

    Sub Main()

        dtDirectors = New DataTable()

        colDirectorID = dtDirectors.Columns.Add()

    End Sub

End Module

If you want to specify the name of the new column when calling the DataColumnCollection.Add() method, use the following version:

Overloads Public Overridable Function Add(ByVal columnName As String) As DataColumn

This method takes as argument the name of the new column and returns that new column. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dtDirectors As DataTable
    Private colDirectorID As DataColumn
    Private colDirector As DataColumn

    Sub Main()

        dtDirectors = New DataTable()

        colDirectorID = dtDirectors.Columns.Add()
        colDirector = dtDirectors.Columns.Add("Director")

    End Sub

End Module

If you have already formally created a DataColumn object, to add it to the collection of columns of a table, call the following version of the DataColumnCollection.Add() method:

Overloads Public Sub Add(ByVal column As DataColumn)

This method expects a DataColumn object as argument. You can either primarily create a DataColumn value or you can define one in the parentheses of the method. Here are two examples:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dtDirectors As DataTable
    Private colDirectorID As DataColumn
    Private colDirector As DataColumn

    Private dtVideos As DataTable
    Private colVideo As DataColumn

    Sub Main()

        dtDirectors = New DataTable()

        colDirectorID = dtDirectors.Columns.Add()
        colDirector = dtDirectors.Columns.Add("Director")

        dtVideos = New DataTable("Videos")
        colVideo = New DataColumn("Video")
        dtVideos.Columns.Add(colVideo)

    End Sub

End Module

1. To create a few columns, change the file as follows:
    

   Imports System Imports System.Xml Imports System.data Class Exercise Private Shared dsVideoCollection As DataSet Private Shared colCategoryID As DataColumn Private Shared colCategory As DataColumn Private Shared tblVideoCategories As DataTable Private Shared tblDirectors As DataTable Private Shared tblRatings As DataTable Private Shared tblActors As DataTable Private Shared tblFormats As DataTable Public Shared Sub Main() dsVideoCollection = New DataSet("Videos") tblDirectors = new DataTable() tblDirectors.TableName = "Directors" dsVideoCollection.Tables.Add(tblDirectors) tblVideoCategories = new DataTable("Categories") dsVideoCollection.Tables.Add(tblVideoCategories) colCategoryID = new DataColumn() colCategoryID.ColumnName = "CategoryID" tblVideoCategories.Columns.Add(colCategoryID) colCategory = new DataColumn() colCategory.ColumnName = "Category" tblVideoCategories.Columns.Add(colCategory) tblRatings = dsVideoCollection.Tables.Add() tblActors = dsVideoCollection.Tables.Add("Actors") tblFormats = dsVideoCollection.Tables.Add("Formats") End Sub End Class

2. Save the file

Instead of adding one column (at a time) to a table, you can first create an array of columns and add that array to the collection of columns. To do this, you can call the DataColumnCollection.AddRange() method. Its syntax is:

Public Sub AddRange(ByVal columns() As DataColumn)

This method takes as argument an array of predefined columns. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dsVideoCollection As DataSet

    Private dtVideos As DataTable
    Private colVideo(7) As DataColumn

    Sub Main()

        dsVideoCollection = New DataSet("Video")

        dtVideos = New DataTable("Video")

        colVideo(0) = New DataColumn("Title")
        colVideo(1) = New DataColumn("Director")
        colVideo(2) = New DataColumn("YearReleased")
        colVideo(3) = New DataColumn("Length")
        colVideo(4) = New DataColumn("Rating")
        colVideo(5) = New DataColumn("Format")
        colVideo(6) = New DataColumn("Category")

        dtVideos.Columns.AddRange(colVideo)

        dsVideoCollection.Tables.Add(dtVideos)

    End Sub

End Module

If you create an object, such as a form, that allows the user to enter records for the above list of videos, you can reduce the likelihood of mistakes. For example, you can prevent the table from allowing the user to enter a director name in a box reserved for the year released. To exercise this type of control, the DataColumn class allows you to specify an appropriate or desired data type for each column.

Just as done in C# regular applications, the data types of a table allow its columns to accept or reject inappropriate values. Although we saw that the name was the most important aspect of a column, in reality, a data type is also required. If you don't specify the data type of a column, it is assumed to be a string and the string data type is automatically applied to it.

To specify the data type of a column, you have two main alternatives. When declaring a column, to specify its data type, you can initialize the DataColumn variable using the third constructor of the class. Its syntax is:

Public Sub New(ByVal columnName As String, ByVal dataType As Type)

To specify a column's data type, select one from the Type class of the System namespace by calling the Type.GetType() method. The GetType() method is overloaded with three versions. The first version has the following syntax:

Overloads Public Shared Function GetType(ByVal typeName As String) As Type

This method expects as argument a valid data type defined in the .NET Framework. The data type must be retrieved from the Type class of the System namespace. The name of the data type must be qualified with a period operator. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dtCategories As DataTable
    Private colCategory As DataColumn

    Sub Main()

        dtCategories = New DataTable("Categories")

        colCategory = New DataColumn("Category", System.Type.GetType("System.String"))
        dtCategories.Columns.Add(colCategory)

    End Sub

End Module

If you used the default constructor to declare a DataColumn, to specify its data type, assign its qualified type to the DataColumn.DataType property. Here is an example:

Imports System
Imports System.Xml
Imports System.Data

Module Exercise

    Private dtCategories As DataTable
    Private colCategoryID As DataColumn
    Private colCategory As DataColumn

    Sub Main()

        dtCategories = New DataTable("Categories")

        colCategoryID = New DataColumn()
        colCategoryID.ColumnName = "CategoryID"
        colCategoryID.DataType = System.Type.GetType("System.Int32")
        dtCategories.Columns.Add(colCategoryID)

        colCategory = New DataColumn("Category", System.Type.GetType("System.String"))
        dtCategories.Columns.Add(colCategory)

    End Sub

End Module

Remember that there are various techniques you can use to create a column by specifying its name and its data type.