9-1

Chapter 9

Working with Databases in VB .NET

9-2

Learning Objectives

Understand how databases are used to store business data and how they differ from arrays.

Describe the parts of a database and understand database operations including working with ADO.NET and the Dataset.

Add oleDBDataAdapter and DataGrid controls to a form and configure them to display records from a database table.

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Learning Objectives (continued)

Add the necessary code to find and display records that match some criteria.

Add programming instructions and controls to edit database records, add new records, delete existing records, and print records in the Dataset.

Use VB .NET and an ADO.NET database to carry out the rental of a DVD.

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Database concepts

Database– The storage of different types of data in a such a way that

the data can be easily manipulated and retrieved by an end user.

Field – A single fact or data item, the smallest unit of named data

that has meaning in a database.– Examples

A name, address, or phone number on a membership list; A product number in an inventory list; or a price in a price list.

– Fields are given field names that are used to identify them.

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More Database Concepts

Record– A collection of related data that is treated as a unit– For example, a membership list record that contains

numerous fields, including name, phone number, street address, city, state, Zip code

Table– A related collection of records, all having the same fields– A table for the Vintage DVDs membership list would have a

record for each person on the list In a table, the records are commonly referred to as

the rows of the table and the fields are the columns of the table.

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Relational Databases

A database with multiple tables that are related is known as a relational database.

The most common type of database used today– MS Access– MS SQL Server

9-7

Relational Database Example

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Primary Key and Foreign Key

Primary Key – A field in a table that uniquely identifies a record in that

same table– In a table containing customers, the Social Security Number

field could be the primary key– No two individuals have the same Social Security Number

Foreign Key– A field in a table that uniquely identifies a record in another

table– In a table containing Payments information, a single field

would be used to keep track of who made the payment.– That field (say Social Security Number) is a foreign key

9-9

Database Operations

The primary use of a database – To enable the user to obtain information from it in a usable

form by constructing queries– For a relational database, these queries are written in a

special language known as SQL (for Structured Query Language),

Queries enable database users to find records from a table that meet some stated criterion or to create composite records from multiple tables that meet the criterion.

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Databases vs Arrays

A database table can store multiple types of data in the same table

Any changes to the database are immediately saved to disk as they occur.

We can use a sophisticated database engine to handle the processing.

It is very easy to connect multiple computers to the same database, so that any changes are immediately known to all computers.

Arrays are restricted to a single data type.

Arrays are inherently volatile, you need to write code to “persist” changes

Array operations all require customized code

No such immediate update, still requires code

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Databases in VB .NET

ADO.Net– Active Data Object .Net– The data access framework used in VB .Net

We use Access as database Similar coding approach would work with any

database providing an OLEDB interface

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Step-by-step 9-1: Starting database project

Demo

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VB .Net Data Objects and Wizards

Data Adapter object– Acts as intermediary between the database on

disk and disconnected in-memory representations of the database called DataSets and DataTables

DataSet – can be made up of one or more database tables

along with their relations DataTable

– One table from the database

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ADO.Net’s way: Disconnected Views

In ADO.Net typical programming, DataSet’s and DataTable’s are disconnected

They are created from the database in computer memory

They are manipulated in memory The results are used to update the original

database on disk when desired

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Data Adapters in VB .Net

Two types of data adapters in VB .NET– oleDbDataAdapter

Designed to work with a number of types of databases including Access databases

The most generic adapter

– sqlDbDataAdapter Specifically designed to work with Microsoft SQL Server

databases The most specific adapter

9-16

Data Objects in the Toolbox

9-17

Configuring a Data Adapter

1. Set up database connection

2. Generate SQL to display fields from a table of the database

3. Generate a Dataset

9-18

Setting up a database Connection

9-19

Setting up a database connection

9-20

Creating the SQL to display fields

9-21

The Query Builder

9-22

Generating a dataset

Two ways to generate a dataset– Click the “Generate Dataset” hyperlink below the

Property window when either the form or data adapter control is selected

– Select the Data|Generate Dataset menu option What it does

– Creates a class from which a DataSet object can be instantiated

– Instantiates an object from the class

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Dataset Generation Dialog Box

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Step-by-Step 9-2: Connecting to the database

Demo

9-25

Displaying records in DataGrid

Add a DataGrid control to project Set the DataSource property of the control

– Use the properties window– At this point, the grid knows the name of the

Fields but not the values of the records

Fill the DataGrid by calling the Fill method of the DataAdapter to which it is “bound”

– oleDBDataAdapter1.Fill(DataSetName,TableName)

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Searching for records

Need an SQL Query– General form

SELECT fieldnames FROM tables WHERE queryORDER BY fieldname;

– Use the * character to indicate all fields SELECT * FROM MyTable

– Not all parts of the general form are necessary

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More SQL

SELECT * FROM Members WHERE Late_Fees > 10– Selects only those members with more than $10 in late fees

SELECT * FROM Members ORDER By Phone_Number

– Order (ascending) the members by their phone number– To obtain a descending order, append DESC at the end

SELECT * FROM Members WHERE Phone_Number = ‘706-555-1234’

– Failure to include the apostrophes will lead to an error – The apostrophes indicate a literal string value in SQL

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Implementing a Search

Clear the Dataset Create the appropriate query string

– Usually on the fly according to some user input

Modify the data adapter to use the new query string rather than the one created with the Query Builder wizard during the data adapter configuration process.

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Code for the Search

Sub procedure to search for late fees

Sub FindLateFees(ByVal decAmount As Decimal) Dim SelString As String DsVintage1.Clear() SelString = "SELECT * “ & _

“FROM Members “ & _“WHERE Late_Fees > Cstr(decAmount)”

MembersAdapter.SelectCommand.CommandText = SelString

MembersAdapter.Fill(DsVintage1, "Members") End Sub

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The LIKE operator

The general form of the query using the LIKE Operator is:– SELECT * FROM – Table WHERE FieldName – LIKE ’%SearchString%’

% is the wild card character for strings % can be replaced by any number of other

characters

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Step-by-Step 9-3: Displaying records in a DataGrid

Demo

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Record Operations

Editing a record in the DataGrid by changing one or more fields and saving the results back to the database

Adding a new record to the database Deleting an existing record Reading records

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Editing records

Editing a record that appears in the DataGrid and saving the changes back to the underlying database table is a two-step process:

1. Make changes directly into the cell in the DataGrid

2. Update these changes to modify the database

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Adding a new record

1. Use a dialog form like that discussed in Chapter 8 to input multiple items of data and transfer them to variables in the btnAdd_Click event procedure in the frmMembers code window

2. Create a new row of the Dataset table composed of the variable contents from Step 1

Make sure no row already exists with same primary key

3. Add the new row to the Dataset table and update the database.

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Adding a Row to DataSet

Sub AddMembers(ByVal strName As String, ByVal strPhoneNumber As String) ‘Declare a DataRow variableDim Member_Row As DataRow

‘Create a new DataRow and assign to the DataRow variableMember_Row = DsVintage1.Tables("Members").NewRow

‘Assign values to each of the fieldsMember_Row("Name") = strNameMember_Row("Phone_Number") = strPhoneNumberMember_Row("Late_Fees") = 0

‘Add the DataRow to the Rows collection of the table DsVintage1.Tables("Members").Rows.Add(Member_Row)

‘Update the database with the changes made to itMembersAdapter.Update(DsVintage1, "Members")

End Sub

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Error Handling: The Try-Catch structure

The complete form of the Try-Catch structure is:

TryProgramming statement to be tested

Catch variable As exception errorProgramming statement to be executed if error is caught

FinallyProgramming statements to be executed in all cases

End Try

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Deleting a record

Private Sub btnDelete_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _Handles btnDelete.Click

Const strDelete As String = "Are you sure you want to delete this record?"

Dim strPhoneNumber As String, intNumRows, intResponse As Integer strPhoneNumber = InputBox("Input phone number of person to be deleted.")

FindPhoneNumber(strPhoneNumber) intNumRows = DsVintage1.Tables("Members").Rows.Count

If intNumRows = 0 Then MsgBox("Nobody with that telephone number!") Exit Sub

End If

intResponse = MsgBox(strDelete, vbYesNoCancel + vbCritical + vbDefaultButton2, "Delete Record")

If intResponse = vbYes Then DsVintage1.Tables("Members").Rows(0).Delete() MembersAdapter.Update(DsVintage1, "members")

Else MsgBox("Record not deleted", vbInformation, "Vintage")

End If End Sub

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FindPhoneNumber

Sub FindPhoneNumber(ByVal strPhNum As String) Dim strSelectString As String DsVintage1.Clear() strSelectString = “SELECT * “ & _

“FROM Members “ &“WHERE " & _ "Phone_Number = '" & strPhNum & "'“

MembersAdapter.SelectCommand.CommandText = _

strSelectString MembersAdapter.Fill(DsVintage1, "Members")

End Sub

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Reading Matching Records

Need two new objects– oleDBCommand– oleDBDataReader

The oleDBCommand control can be dragged from the toolbox

Use the oleDBCommand objet to generate the oleDBDataReader

The oleDBDataReader is used to simply read through a Dataset without making any changes

9-40

Code to read matching records

Private Sub btnPrint_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnPrint.Click

Dim datReader As OleDb.OleDbDataReader, strOutput As String MembersConnection.Open() ‘MembersCommand is an OleDBCommand objectdatReader = MembersCommand.ExecuteReader

While datReader.Read() strOutput = datReader("Name").ToString() _

& vbTab & datReader("Phone_Number").ToString() _ & vbTab & _Format(datReader("late_fees").ToString, "currency")

Debug.WriteLine(strOutput) End While

End Sub

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Step-by-Step 9-4: Record Operations

Demo

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Step-by-Step 9-5: Renting a DVD

Demo

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Copyright 2004 John Wiley & Sons, Inc.

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