Check if record already exists - mysql

Question

I am creating a program in VB.NET where, before creating a user I want to check that if that record already exists.
Here is my code:
Imports MySql.Data.MySqlClient
Public Class WindowsAdvancedStudyStartingForms
Dim FirstName As String = ""
Dim SecondName As String = ""
Dim FullName As String = ""
Dim StudentClassReal As String = ""
Dim StudentClassValue As String = ""
Dim Address As String = ""
Dim Username As String = ""
Dim Password As String = ""
Dim SuccessfulMessage As Integer = 0
'MySql
Dim ServerString As String = "Server=myServer;User ID=myID;Passwordmy=Password;Database=myDatabase;SSLMode=None"
Dim SQLConnection As MySqlConnection = New MySqlConnection
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
FirstName = AFirstNameTextBox.Text
SecondName = ASecondNameTextBox.Text
FullName = FirstName + " " + SecondName
StudentClassValue = ASelectClassComboBox.SelectedItem
Address = AAddressTextBox.Text
Username = AUsernameTextBox.Text
Password = APasswordTextBox.Text
If StudentClassValue = "Class IX" Then
StudentClassReal = 9
Else
MessageBox.Show("You have selected a Wrong Class", "Wrong Class", MessageBoxButtons.OK, MessageBoxIcon.Error)
End If
If FirstName = "" And FirstName.Count = 1 Then
MessageBox.Show("You didn't enter your First Name", "First Name", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
'Nothing
End If
If SecondName = "" Then
MessageBox.Show("You didn't enter your Second Name", "Second Name", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
'Nothing
End If
If Address = "" Then
MessageBox.Show("You didn't enter your Address", "Address", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
'Nothing
End If
If Username = "" Then
MessageBox.Show("You didn't enter your Username", "Username", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
'Nothing
End If
If Password = "" Then
MessageBox.Show("You didn't enter your Password", "Password", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
'Nothing
End If
If StudentClassReal = "9" Then
Dim StudentInformationVerification As Integer = MessageBox.Show("Are you sure that these are your information?" & vbCrLf & "I am " + FullName + ", and I study at Class " + StudentClassReal + ". I live in " + Address + ". My Advanced Windows Study Username is " + Username + ", and my password is " + Password, "Information Verification", MessageBoxButtons.YesNo, MessageBoxIcon.Question)
If StudentInformationVerification = DialogResult.Yes Then
Dim SQLStatement As String = "INSERT INTO people(FirstName, SecondName, Class, Address, Username, Password) VALUES('" & FirstName & "','" & SecondName & "', '" & StudentClassReal & "', '" & Address & "', '" & Username & "', '" & Password & "')"
SaveName(SQLStatement)
My.Computer.Registry.LocalMachine.SetValue("Study", "1")
SuccessfulMessage = 1
Me.Close()
End If
If StudentInformationVerification = DialogResult.No Then
End If
Else
'Nothing
End If
End Sub
'MYSQL Connection
Private Sub WindowsAdvancedStudyStartingForms_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim RegistryCheck As String = My.Computer.Registry.LocalMachine.GetValue("Study")
If RegistryCheck = 1 Then
LoginForm1.Show()
Me.Close()
End If
APasswordTextBox.UseSystemPasswordChar = True
SQLConnection:
SQLConnection.ConnectionString = ServerString
Try
If SQLConnection.State = ConnectionState.Closed Then
SQLConnection.Open()
If SuccessfulMessage = 0 Then
MessageBox.Show("Connected to Windows Advanced Study Database", "Connection to Database Passed", MessageBoxButtons.OK, MessageBoxIcon.Information)
End If
Else
SQLConnection.Close()
MessageBox.Show("Could not Connect to Windows Advanced Study Database", "Connection to Database Failed", MessageBoxButtons.RetryCancel, MessageBoxIcon.Error)
If DialogResult.Retry Then
GoTo SqlConnection
End If
If DialogResult.Cancel Then
Close()
End If
End If
Catch ex As Exception
MsgBox(ex.ToString)
End Try
End Sub
Public Sub SaveName(ByRef SQLStatement As String)
Dim cmd As MySqlCommand = New MySqlCommand
With cmd
.CommandText = SQLStatement
.CommandType = CommandType.Text
.Connection = SQLConnection
.ExecuteNonQuery()
End With
MessageBox.Show("Welcome to Advanced Windows Studying", "Authentication Successful", MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub
End Class

Answer 1

Use parameters. Turn on Option Strict.
You won't need all those Dim's for the student info.
You can pass your connection string to the constructor of the MySQLConnection.
Move your validation code to the Validat event of the various controls. Look up how to cancel the event so focus stays on the appropriate control.
You can pass your SQL string and the connection directly to the connection constructor.
Using...EndUsing statements will ensure that objects are closed and disposed properly even if there is an error. This is very important to ensure that connections are closed.
Don't use GoTo. This is only in the language for backward compatibility and should not be used in new code. (leads to spaghetti code)
In a real application, passwords would never be stored as plain text but that is a topic for another day.
In your SQL statements you need to surround and MySQL reserved words with back ticks (`) This is the character below the tilde (~) and is the quoted identifier for MySQL. Actually, it doesn't hurt to surrond all table names and column names with the back tick to be safe.
I cannot test this code because I don't have your database.
Imports MySql.Data.MySqlClient
Public Class MySQLStudent
Dim strConnection As String = "Server=myServer;User ID=myID;Passwordmy=Password;Database=myDatabase;SSLMode=None"
Private Sub RegisterStudent()
Using cn As New MySqlConnection(strConnection)
Dim SQLStatement As String = "Select Count(*) From people where Username = #UserName;"
Using cmdV As New MySqlCommand(SQLStatement, cn)
cn.Open()
Dim rowCount As Integer = CInt(cmdV.ExecuteScalar())
cn.Close()
If rowCount > 0 Then
MessageBox.Show("Sorry that username is in use; please enter another one.")
AUsernameTextBox.Focus
Exit Sub
End If
End Using
SQLStatement = "INSERT INTO people(FirstName, SecondName, `Class`, Address, `Username`, `Password`) VALUES(#FirstName, #SecondName', #StudentClassValue, #Address, #UserName, #Password);"
Using cmd As New MySqlCommand(SQLStatement, cn)
cmd.Parameters.Add("#FirstName", MySqlDbType.String).Value = AFirstNameTextBox.Text
cmd.Parameters.Add("#SecondName", MySqlDbType.String).Value = ASecondNameTextBox.Text
cmd.Parameters.Add("#StudentClassValue", MySqlDbType.String).Value = ASelectClassComboBox.SelectedItem
cmd.Parameters.Add("#Address", MySqlDbType.String).Value = AAddressTextBox.Text
cmd.Parameters.Add("#Username", MySqlDbType.String).Value = AUsernameTextBox.Text
cmd.Parameters.Add("#Password", MySqlDbType.String).Value = APasswordTextBox.Text
cn.Open()
With cmd
.CommandType = CommandType.Text
.ExecuteNonQuery()
End With
End Using
End Using
MessageBox.Show("Welcome to Advanced Windows Studying", "Registration Successful", MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub
Private Sub LogInStudent()
Using cn As New MySqlConnection(strConnection)
Using cmd As New MySqlCommand("Select Count(*) From people Where `Username` = #UserName And `Password` = #Password;", cn)
cmd.Parameters.Add("#UserName", MySqlDbType.String).Value = AUsernameTextBox.Text
cmd.Parameters.Add("#Password", MySqlDbType.String).Value = APasswordTextBox.Text
cn.Open()
Dim rowCount As Integer = CInt(cmd.ExecuteScalar)
cn.Close()
If rowCount <> 1 Then
MessageBox.Show("Sorry, invalid login.")
Exit Sub
End If
MessageBox.Show("Successful login.")
'Show the next form of the application
End Using
End Using
End Sub
End Class