Checking if database values already exist in mysql

[sangguko19]

Hello Good Day this time my only problem is how to check if the specific data value from database is already exist. I have a system which is student attendance and I also have a database table for students that contains a values studonsite, This studonsite value will be updated every student scanned depends on the student id tag, if one student tag is scanned the studonsite table will turn to 1 represented as the Student is inside the campus and 0 is OUT. The problem is If the student scanned his ID tag twice in the IN which is '1' without scanning in the out (0) the warning will pop up that the student is already login, and he must log out first.

Here is my code for update

    Public Sub updatein()
        con = New MySqlConnection
        con.ConnectionString = "server=localhost;userid=root;password=1234;database=dat"
        Dim reader As MySqlDataReader

        Try
            con.Open()
            Dim query As String

            query = "update dat.students set studonsite = '1' where studtags='" & studtag.Text & "' "
            cmd = New MySqlCommand(query, con)
            reader = cmd.ExecuteReader


            con.Close()
        Catch ex As MySqlException
            MessageBox.Show(ex.Message)
        Finally
            con.Dispose()
        End Try



    End Sub

And for the RFID tag Textbox event

    Private Sub studtag_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles studtag.TextChanged

        Notenrolled.Close()
        greet.Text = ""
        PictureBox4.Visible = False

        If studtag.Text = "44F2F38B" Then

            Endday()

        End If
        


        If studtag.Text = recents.Text Then

            alreadylogin.Show()
        ElseIf studtag.TextLength = 8 Then

            DataGridView1.Sort(DataGridView1.Columns(6), System.ComponentModel.ListSortDirection.Descending)

            Dim table As New DataTable

            Try
                con.Open()
                'Gets or sets an SQL statement or stored procedure used to select records in the database.
                With cmd
                    .Connection = con
                    .CommandText = "SELECT * from students where `studtags`='" & studtag.Text & "';"
                End With
                da.SelectCommand = cmd
                da.Fill(table)

                

                idno.Text = table.Rows(0).Item(1)
                lastxt.Text = table.Rows(0).Item(2)
                firstxt.Text = table.Rows(0).Item(3)
                middletxt.Text = table.Rows(0).Item(4)
                dob.Text = table.Rows(0).Item(6)
                year.Text = table.Rows(0).Item(7)
                crsetxt.Text = table.Rows(0).Item(10)

                tagtxt.Text = studtag.Text
                timein.Text = times.Text

                If studtag.Text = table.Rows(0).Item(17) Then
                    greets.Text = "You already login"
                End If


                dr = cmd.ExecuteReader()
                dr.Read()

                If dob.Text = bday.Text Then
                    greet.Text = firstxt.Text + " Today is your Birthday. Greetings :D."
                    PictureBox4.Visible = True

                End If

                Dim img() As Byte = CType(dr("studpic"), Byte())


                Using ms As New IO.MemoryStream(img)
                    PictureBox1.Image = Image.FromStream(ms)

                End Using
                '--> uses relative path here
                My.Computer.Audio.Play("C:\Users\BOR\Desktop\Parsu Gate\Parsu\audio\scanned.wav")
                insert()
                loadtable()
                studdailyhistory()
                updatein()

            Catch ex As Exception
                MessageBox.Show(ex.Message)
                Notenrolled.Show()
                DataGridView1.Sort(DataGridView1.Columns(5), System.ComponentModel.ListSortDirection.Descending)
                If studtag.Text = "44F2F38B" Then
                    Notenrolled.Close()
                End If


            Finally

                con.Dispose()
                con.Close()

            End Try

        End If
        recents.Text = tagtxt.Text



    End Sub

I wonder that I can use If statement but I failed Here is my start

    If studtag.Text = table.Rows(0).Item(17) = 1 Then
                    greets.Text = "You already login"
                End If

My goal in this function is :
If the studtag.text(where the RFID tag displayed) which has studonsite values '1' then the message displayed

I will do any suggestions, Thank you so much.