how to detect wrong input from user vb - mysql

Question

I want my system to detect wrong input from the user . for example I use " matric_no " as the primary key in my database and data for the primary key is a " D1233455 " . when the user wants to delete a record in the database , they must type the primary key and then pressing the delete button . but if the primary key is entered incorrectly , the system will continue to detect errors when the delete button is pressed , and I do not have to restart my program to correct the input.I used the coding to link the database.
but the problem is everything that i explained above not happened..i have to restart my system to enter new correct input. i hope someone can help me..The most important is i want to know how to detect wrong input from user. i already found how to enter new input without restarting my system...help me please..
Dim query As String = "delete from Student_Database where Matric_No = '" & TextBox1.Text & "'"
Dim query1 As String = "delete from Fee_Database where Matric_No = '" & TextBox1.Text & "'"
If Me.TextBox1.Text = String.Empty Then
Notify the user of the invalid value.
MessageBox.Show("Please enter a value.", _
"Required Field", _
MessageBoxButtons.OK, _
MessageBoxIcon.Warning)
ElseIf ("I need the solution for this part") Then
MessageBox.Show("Record cannot be trace. Please enter the correct ID", "Required Field", MessageBoxButtons.OK, MessageBoxIcon.Warning)
Else
objResult.queryCommand(query)
objResult.queryCommand(query1)
MessageBox.Show("Record has been deleted.", "Info", MessageBoxButtons.OK, MessageBoxIcon.Information)
End If

Answer 1

Is there a specific syntax of how you want your value to be. You have mentioned it as "D1233455" and if it is always in that format, then you can just compare the first character of the input and count the the length of the input which should be 8. If these conditions are false then it should show a message box stating the value is invalid.

Answer 2

You need a select query that select all the matric_no rows, then compare it with value on your textbox.
And in case that I'm using the MS-Access database, so I use OleDb connection, if you're using another database, just change the connection to what you use.
Here the connection what I use
Imports System.Data.OleDb
Module Connection
Public Conn As New OleDbConnection
Public CMD As OleDbCommand
Public DA As OleDbDataAdapter
Public DR As OleDbDataReader
Public DS As DataSet
Public DT As DataTable
Public STR As String
Public Sub Connect()
Try
STR = "provider=microsoft.ACE.OLEDB.12.0;data source=" & Application.StartupPath & "\your_database_name_here.mdb"
Conn = New OleDb.OleDbConnection(STR)
If Conn.State = ConnectionState.Closed Then
Conn.Open()
End If
Catch ex As Exception
MsgBox("Failed to connect database")
End Try
End Sub
End Module
Then here the code for your problem
'Add this to the top of the code
'Actually before the `Public Class <your_form_name>`
'So it's like
Imports System.Data.OleDb
Public Class blablabla
.....
Private Sub DeleteData()
Dim query As String = "delete from Student_Database where Matric_No = '" & TextBox1.Text & "'"
Dim query1 As String = "delete from Fee_Database where Matric_No = '" & TextBox1.Text & "'"
'Changes
Dim query2 As String = "SELECT * FROM Student_Database where Matric_No = '" & TextBox1.Text & "'" 'This query to detect that user input is same to any Matric_No rows.
Connect()
CMD = New OleDbCommand(query2, Conn)
DR = CMD.ExecuteReader
DR.Read
If DR.HasRows Then
If Me.TextBox1.Text = String.Empty Then
Notify the user of the invalid value.
MessageBox.Show("Please enter a value.", _
"Required Field", _
MessageBoxButtons.OK, _
MessageBoxIcon.Warning)
Else
objResult.queryCommand(query)
objResult.queryCommand(query1)
MessageBox.Show("Record has been deleted.", "Info", MessageBoxButtons.OK, MessageBoxIcon.Information)
End If
Else 'Here you detect the wrong input
MessageBox.Show("Record cannot be trace. Please enter the correct ID", "Required Field", MessageBoxButtons.OK, MessageBoxIcon.Warning)
End If
Conn.Close()
End Sub
Hope those help you.