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Lesson 6 — Database Programming

Microsoft Visual Basic .NET, Introduction to Programming

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Objectives

Explain what a database is and how to use it. Build an ODBC Data Source Name from an Access

database. Link a database to a Visual Basic application. Design an application to display records of a

database using the DataGrid control. Design an application to display a single record of a

database by binding the fields of the database to text boxes.

Apply user-defined data types (UDTs) in an application.

Use a sequential file to store data from an array of a user-defined data type.

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Vocabulary

Application programming interface (API)

Data Source Name (DSN)

Database DataBindings property DataGrid DataSet

DataSource property Fields Open Database

Connectivity (ODBC) Record Structured query

language (SQL) Structures User-defined data type

(UDT)

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Databases

A database is an organized collection of related data. Data are facts, names, dates, and values. Once data is organized, it becomes information. An example of data that is not information is a list of telephone numbers that isn't alphabetized. The lack of organization makes it difficult to find a number.

Databases are organized in tables. A database containing information about running a school might have individual tables containing data on teachers, course offerings, classrooms, class lists, and students enrolled.

Within each table, data is organized into records. Each record is one complete entry. In the teacher table, for example, a record exists for each teacher employed by the school.

Records are further subdivided into fields.

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Note

As a programmer, you might know little or nothing about the information for which you are designing a database. You might be asked to design a database about experimental cancer drugs or performance camshafts. If you know nothing about the information, you will be dependent on the users to help you determine what is important and what is not. This is one reason that communication with the users is so important.

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Preparing the Database

You must prepare a database before accessing it with a Visual Basic program. An application programming interface (API) called Open Database Connectivity (ODBC) is commonly used to prepare a database for use in a program. APIs provide ways for programmers and users to utilize functions built into the operating system. ODBC makes it possible for applications to access the information stored in a database.

To use ODBC, you must create a Data Source Name (DSN) for the database.

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The ODBC Data Source Administrator

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Datasets

A dataset is the storage of copies of some or all of the records of the database. It may combine records or fields from various tables in a single database or information from several databases.

Why not access a database directly from an application? Today, databases may be stored on network servers or on the Internet. To maintain a continuous connection from the user's computer to the database could consume a great many network resources. It is also likely that others will need access to the same database at the same time. To facilitate this sharing of databases and to limit the use of network resources, a portion of the data is read from the database and stored in a dataset.

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Data Adapter Configuration Wizard

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Choose Your Data Connection Dialog Box

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Data Link Properties Dialog Box Set for SQL Server

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Data Link Dialog Box Set for an Access Database

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Choosing a Query Type

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Generating the SQL Statements

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Query Builder Dialog Box

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View Wizard Results Pane of the Data Adapter Configuration Wizard

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Generate Dataset Class Dialog Box

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Step-by-Step 6.1

OleDbDataAdapter1.Fill(DsPortableCD1)

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The Choosing a CD Player Program

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DataGridView Control

The DataGridView control is a wonder of automation. Once linked to a properly configured dataset, it automatically identifies the fields and creates a column for each field. If the dataset contains more than one table, the data grid automatically changes to allow the display of each table.The tables then have a hierarchical arrangement, in which one table is the parent to the other tables. A number of the properties of the DataGrid control relate to the parent table.

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DataGrid Control

The columns created by the DataGrid control are called DataGridColumns. As each column is created, it is added to the GridColumns collection. The data grid creates a new row for each record and automatically provides a header for each column that contains the name of the field. You can modify the properties of a column in the DataGridColumn Collection Editor. Likewise, you can add or delete columns in the DataGridColumn Collection Editor.

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Displaying a Single Record on a Form

It is possible to bind the fields of a table of a database to text boxes. The text boxes then provide a place in which to display and edit the values in the fields. To make a connection between a particular field of a dataset and a text box, you use the text box’s DataBindings property.

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DataBindings Property

When you expand the DataBindings property and click the Advanced button, the Advanced Data Binding dialog box opens. The Advanced Data Binding dialog box lists many of the properties of the text box. Each property may be bound to a field of the dataset. Typically, to display the information in a database, you bind a field of the dataset to the Text property of the text box. It is a simple matter to envision a one-to-one correspondence between the fields of a dataset and text boxes on the form.

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Displaying Data

Setting up an application to display data in text boxes instead of a data grid requires a little more work.

First, you populate the form with text boxes and corresponding labels.

Next, you bind each text box to a field of the dataset.

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CurrencyManager Object

Each field of the dataset represents a column of data. Multiple textboxes linked to the same data source and each linked to a particular field of the dataset must be managed so the data they display is coordinated. For instance, if one text box is bound to the last name and one to the first name, it would not do to display the first name of one record with the last name of a different record. To coordinate the display of information, each data source has a CurrencyManager object. The CurrencyManager object coordinates the display of multiple text boxes linked to different fields of the same dataset. If there is more than one data source bound to a form, each has its own CurrencyManager.

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BindingContext Object

To manage multiple CurrencyManagers, each form has a BindingContext object. The BindingContext object uses two parameters: the first is the name of the dataset, and the second is the name of the table within the dataset to which the data belongs. In the simplest case, the form’s BindingContext object has a single CurrencyManager object that coordinates the display of the data on the form.

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Data Adapter Preview Window

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The SingleRecord Application

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Binding the Manufacturer Field of the Dataset to the Text Property

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Advanced Data Binding Dialog Box

Binding the Model field of the dataset to the Text property

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Step-by-Step 6.3

With Me.BindingContext(dsPortableCD1, "Portable CD Players")

If .Position > 0 Then

.Position -= 1

End If

End With

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Step-by-Step 6.3

Dim iCnt As IntegerWith Me.BindingContext(dsPortableCD1, "Portable

CD Players") iCnt = .Count - 1 If .Position < iCnt Then .Position += 1 End IfEnd With

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User-defined Data Types

As versatile as the various data types are, it is nevertheless often useful to design your own data types from other built-in types. In Visual Basic, these data types are called structures or user-defined data types (UDTs). Structures are useful for representing data that is too complex to represent with a single simple data type.

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Structure Statement

You define structures by using a Structure statement. A simple example follows:

Structure AutomobilePublic Make As StringPublic Model As StringPublic Price As Decimal

End Structure

The name of the data type, Automobile, follows the keyword Structure. Declarations for the fields of the structure appear between the Structure and End Structure statements. The fields are the individual parts that make up the structure. Each field, in turn, has its own data type.

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Layout of Controls

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Step-by-Step 6.4

Structure Automobile

Public Make As String

Public Model As String

Public Price As Decimal

End Structure

Const Capacity As Integer = 50

Public CarLot(Capacity) As Automobile

Public LotNumber As Integer = 0

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Step-by-Step 6.4

If LotNumber = 0 Then Exit SubEnd IfDim Index As IntegerFor Index = 0 To LotNumber - 1 With CarLot(Index) .Make = "" .Model = "" .Price = 0 End WithNextLotNumber = 0lstCarLot.Items.Clear()

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Step-by-Step 6.4

txtMake.ResetText()

txtModel.ResetText()

txtPrice.ResetText()

txtMake.Focus()

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Step-by-Step 6.4

If txtMake.Text.ToString = "" Or txtModel.Text.ToString = "" Or _ CDec(txtPrice.Text) = 0 Then MessageBox.Show("Please fill all the fields.") Exit SubEnd IfIf LotNumber = Capacity Then MessageBox.Show("The lot is full.") Exit SubEnd If

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Step-by-Step 6.4

With CarLot(LotNumber)

.Make = txtMake.Text.ToString

.Model = txtModel.Text.ToString

.Price = CDec(txtPrice.Text)

End With

LotNumber += 1

miClear_Click(sender, e)

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Step-by-Step 6.4

Dim Index As IntegerlstCarLot.Items.Clear()For Index = 0 To LotNumber - 1 With CarLot(Index) lstCarLot.items.add(.Make & " " & .Model & "

" & .Price.ToString) End WithNext

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Step-by-Step 6.4

Dim PathName As String

With OpenFileDialog1

.DefaultExt = "txt"

.InitialDirectory = "c:\My Documents"

.Filter = "Text files | *.txt"

.ShowDialog()

PathName = .FileName

End With

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Step-by-Step 6.4

Dim fs As StreamReaderfs = File.OpenText(PathName)LotNumber = CInt(fs.ReadLine)Dim Index As IntegerFor Index = 0 To LotNumber - 1 With CarLot(Index) .Make = fs.ReadLine .Model = fs.ReadLine .Price = CDec(fs.ReadLine) End WithNextfs.close()miDisplay_Click(sender, e)

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Step-by-Step 6.4

Dim PathName As String

With SaveFileDialog1

.DefaultExt = "txt"

.InitialDirectory = "c:\My Documents"

.Filter = "Text files | *.txt"

.ShowDialog()

PathName = .FileName

End With

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Step-by-Step 6.4

Dim fs As StreamWriterfs = File.CreateText(PathName)Dim strContent As String = LotNumber.ToStringfs.WriteLine(strContent)Dim Index As IntegerFor Index = 0 To LotNumber - 1 With CarLot(Index) fs.WriteLine(.Make) fs.WriteLine(.Model) fs.WriteLine(.Price.ToString) End WithNextfs.Close()

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Imports System.IO

Enable the file statements used in the earlier steps to work by inserting the following statement above Public Class Form1. It brings the File I/O (Input/Output) objects into the class definition of the form. It should be the first line in the Code window.

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Summary

A database is an organized collection of related data. It is divided into tables, records, and fields. Each field holds an individual piece of data, such as a name or a value. The design of the database is important because an appropriate design makes it easier to access the information contained in the database.

An application programming interface (API) called Open Database Connectivity (ODBC) is used to make a database available for use in an application. To use ODBC, you create a Data Source Name (DSN) for the database through the ODBC Data Source Administrator in the Control Panel.

A dataset is a collection of copies of some or all of the records of a database.

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Summary

You use structured query language (SQL) to write statements that select records from a database.

You use a DataGrid control to display the records of a dataset. Once connected to the dataset, the data grid configures itself to display each field of each record in the dataset.

To establish a connection between an application and a local database, put an OleDbDataAdapter on the form. The Data Adapter Wizard guides you through the process of establishing a connection and creating the commands to create a dataset.

You use the DataBindings property to link a control to a field of a dataset.

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Summary

A structure, or user-defined data type (UDT), provides a custom data type created from other built-in types or other structures. The parts of a structure are called fields. Variables of the data type defined by the structure can be declared in procedures or at the module level. Fields are accessed with the dot notation Variablename.fieldname.

Variables declared with the Public keyword in the Declarations section (the top of the Form class definition) are visible throughout the project.