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 Microsoft Visual Studio .NET and create a new Windows Forms Application named PeopleInMyLife3
2. In the Toolbox, click the Data button.
   To create a DataSet, click DataSet and click the form
    
   
3. In the Add Dataset dialog box, click the Untyped Dataset radio button and click OK
4. While the DataSet control is still selected, in the Properties window, click (Name) and type dsPeople
5. Still in the Properties window, set the DataSetName to People
6. As done for the lesson on tables, under the form, click dsPeople if necessary. In the Properties window, click the Tables field to reveal its ellipsis button
7. Click the ellipsis button
8. In the Tables Collection Editor, click the Add button and change its properties as follows:
   (Name): tblPersons
   TableName: Persons
9. Click the Add button again and set the properties as follows:
   (Name): tblGenders
   TableName: Genders
10. Click the Add button again and set the properties as follows:
    (Name): tblTypesOfRelationship
    TableName: TypesOfRelationship
     
    
11. Click Close

A column of a table is an object of type DataColumn. Therefore, to create a column, you can first declare a pointer to 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:

System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 DataColumn *colCategoryID = new DataColumn;
}

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:

private:
	DataColumn *colCategory;	


System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 colCategory = 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:

System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{			
	 colCategory = new DataColumn(S"Category");
}

If you have already declared a DataColumn object, to specify or change its name, assign the desired string to the DataColumn::ColumnName property. 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. As mentioned earlier, 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:

public: virtual DataColumn* Add();

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:

private:
	DataSet *dsVideoCollection;

	DataTable *dtDirectors;
	DataColumn *colDirector;	


System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");

	 dtDirectors = new DataTable();
	 dtDirectors->TableName = S"Directors";
	 colDirector = dtDirectors->Columns->Add();
	 colDirector->ColumnName = S"Director";

	 dsVideoCollection->Tables->Add(dtDirectors);
}

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

public: virtual DataColumn* Add(String *name);

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

private:
	DataSet *dsVideoCollection;

	DataTable *dtDirectors;
	DataColumn *colDirector;	


System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");

			
	 dtDirectors = new DataTable(S"Directors");
	 colDirector = new DataColum();
	 colDirector->ColumnName = S"Director";
	 colDirector = dtDirector->Columns->Add("Director");
	dsVideoCollection->Tables->Add(dtDirectors);			 
}

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:

public: virtual void* Add(DataColumn *name);

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:

private:
	DataSet *dsVideoCollection;

	DataTable *dtVideos;
	DataColumn *colVideo;	


System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");
	 
	 dtVideos = new DataTable(S"ListOfVideos");
	 colVideo = new DataColumn(S"Title");
	 dtVideos->Columns->Add(colVideo);
	 dtVideos->Columns->Add(new DataColumn(S"Director"));
	 dsVideoCollection->Tables->Add(dtVideos);
}

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: void AddRange(DataColumn*columns[]);

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

private:
	DataSet *dsVideoCollection;

	DataTable *dtVideos;
	static DataColumn *colVideos __gc[] = __gc new DataColumn*[7];
				 
	. . . No Change


System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");

	 dtVideos = new DataTable(S"Videos");
		
	 colVideos[0] = new DataColumn(S"Title");
	 colVideos[1] = new DataColumn(S"Director");
	 colVideos[2] = new DataColumn(S"YearReleased");
	 colVideos[3] = new DataColumn(S"Length");
	 colVideos[4] = new DataColumn(S"Rating");
	 colVideos[5] = new DataColumn(S"Format");
	 colVideos[6] = new DataColumn(S"Category");

	 dtVideos->Columns->AddRange(colVideos);

	 dsVideoCollection->Tables->Add(dtVideos);
}

1. Under the form, click dsPeople if necessary. In the Properties window, click the Tables field to reveal its ellipsis button and click the ellipsis button
2. In the Members list, click Genders if necessary. In the Genders Properties list, click the Columns field to reveal its ellipsis button and click its ellipsis button
3. In the Columns Collection Editor, click the Add button
4. While Column1 is still selected in the Members list, in the Column1 Properties list, click (Name) and type colGender
5. Click ColumnName and type Gender
    
   
6. Click Close
7. In the same way, create the following columns for the corresponding tables
    

   Table	Column (Name)	ColumnName
   TypesOfRelationship	colTypeOfRelationship	TypeOfRelationship
   Persons	colFullName	FullName
   Persons	colFriendlyName	FriendlyName
   Persons	colRelationshipType	TypeOfRelationship
   Persons	colKnownSince	KnownSince
   Persons	colCurrentResidence	CurrentResidence
   Persons	colPhoneNumber	PhoneNumber
   Persons	colLastUpdate	LastUpdate
   Persons	colNotifyIfEmergency	NotifyIfEmergency

8. Click Close twice

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. Fortunately, 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 assigned 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. Its syntax is:

public: DataColumn(String* name, Type dataType);

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:

public: static Type* GetType(String* typeName);

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:

System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 colCategory   = new DataColumn(S"Category", System::Type::GetType(S"System.String"));
}

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:

System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");

	 dtDirectors = new DataTable;
	 dtDirectors->TableName = S"Directors";
	 colDirector = new DataColumn();
	 colDirector->ColumnName = S"Director";
	 colDirector->DataType = System::Type::GetType(S"System.String");
	 colDirector = dtDirectors->Columns->Add("Director");
	 dsVideoCollection->Tables->Add(dtDirectors);
}

Remember that there are various techniques you can use to create a column by specifying its name and its data type. Here are various examples from what we have used so far:

#pragma once


namespace ExoTables1
{
	using namespace System;
	using namespace System::ComponentModel;
	using namespace System::Collections;
	using namespace System::Windows::Forms;
	using namespace System::Data;
	using namespace System::Drawing;

	. . . No Change

private:
	
	System::ComponentModel::Container * components;
	DataSet *dsVideoCollection;

	DataTable *dtVideoCategories;
	DataColumn *colCategory;

	DataTable *dtDirectors;
	DataColumn *colDirector;

	DataTable *dtActors;
	DataColumn *colActor;

	DataTable *dtVideos;
	static DataColumn *colVideos __gc[] = __gc new DataColumn*[7];

	. . . No Change

	
System::Void Form1_Load(System::Object *  sender, System::EventArgs *  e)
{
	 dsVideoCollection = new DataSet(S"Videos");

	 dtVideoCategories = new DataTable(S"Categories");
	 colCategory = new DataColumn(S"Category", System::Type::GetType(S"System.String"));
	 dsVideoCollection->Tables->Add(dtVideoCategories);

	 dtDirectors = new DataTable;
	 dtDirectors->TableName = S"Directors";
	 colDirector = new DataColumn();
	 colDirector->ColumnName = S"Director";
	 colDirector->DataType = System::Type::GetType(S"System.String");
	 colDirector = dtDirectors->Columns->Add("Director");
	 dsVideoCollection->Tables->Add(dtDirectors);

	 dtActors = dsVideoCollection->Tables->Add(S"Actors");
	 colActor = new DataColumn(S"Actor", System::Type::GetType(S"System.String"));
	 dtActors->Columns->Add(colActor);
	 
	 dtVideos = new DataTable(S"ListOfVideos");
		
	 colVideos[0] = new DataColumn;
	 colVideos[0]->ColumnName = S"Title";
	 colVideos[0]->DataType = System::Type::GetType(S"System.String");
	 colVideos[1] = new DataColumn(S"Director");
	 colVideos[1]->DataType = System::Type::GetType(S"System.String");
	 colVideos[2] = new DataColumn(S"YearReleased", System::Type::GetType(S"System.Int16"));
	 colVideos[3] = new DataColumn(S"Length", System::Type::GetType(S"System.String"));
	 colVideos[4] = new DataColumn(S"Rating", System::Type::GetType(S"System.String"));
	 colVideos[5] = new DataColumn(S"Format", System::Type::GetType(S"System.String"));
	 colVideos[6] = new DataColumn(S"Category", System::Type::GetType(S"System.String"));

	 dtVideos->Columns->AddRange(colVideos);
	 dsVideoCollection->Tables->Add(dtVideos);
}
};
}

1. Under the Data Center form, click dsPeople if necessary. In the Properties window, click the Tables field to reveal its ellipsis button and click the ellipsis button
2. In the Members list, click Persons if necessary. In the Persons Properties list, click the Columns field to reveal its ellipsis button and click its ellipsis button
3. In the Members list of the Columns Collection Editor, click LastUpdate
4. In the LastUpdate Properties list, click DataType to display the arrow of its combo box and select System.DateTime
    
   
5. In the Members list, click NotifyIfEmergency and, in the NotifyIfEmergency Properties list, click DataType and set it to System.Boolean
6. Click Close twice