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Question

I'm doing a project for my Database Management subject. I cannot figure out how to add an amount to a previously added amount. For now, I'm only able to update the amount. Here's the code. I'm sorry if I cannot explain it well.
I have 2 forms. My first form allows me to enter a last name and retrieve the data to my list view.
My second form lets me retrieve the data I entered in my first form and it will show up on a separate list view with a "Last Name | Amount" tab.
I have two textboxes. One for last name set to readonly to disable editing, and another for the amount I want to enter.
After entering an amount, let's say 20, it will update on the listview and my database as 20.
The problem is that when I enter a new amount for the same last name, let's say 30, the 30 will replace the 20 but it should be 50 because 20+30 = 50.
I understand the logic and I have tried adding another textbox for addition but I simply do not know the codes for it.
Imports System.Data.OleDb
Public Class Form2
Dim conString As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Israel De Leon\Documents\testing.accdb;"
Dim con As OleDbConnection = New OleDbConnection(conString) 'Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\Database2.accdb
Dim cmd As OleDbCommand
Dim adapter As OleDbDataAdapter
Dim dt As DataTable = New DataTable()
Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'SET LISTVIEW PROPERTIES
ListView1.View = View.Details
ListView1.FullRowSelect = True
'Construct Columns
ListView1.Columns.Add("Last Name", 100)
ListView1.Columns.Add("Amount", 100)
End Sub
Private Sub UpdateLV(lname As String)
'Updates last name and amount entered into the database
Dim sql As String = "UPDATE Table1 SET LastName='" + TextBox1.Text + "',Amount='" + TextBox2.Text + "' WHERE LastName='" + lname + "'"
cmd = New OleDbCommand(sql, con)
'OPEN CON, EXECUTE, UPDATE, CLOSE
Try
con.Open()
adapter = New OleDbDataAdapter(cmd)
adapter.UpdateCommand = con.CreateCommand()
adapter.UpdateCommand.CommandText = sql
If (adapter.UpdateCommand.ExecuteNonQuery() > 0) Then
MsgBox("Successfully Updated")
End If
con.Close()
Retrieve()
ClearBox()
Catch ex As Exception
MsgBox(ex.Message)
con.Close()
End Try
End Sub
Private Sub Retrieve()
ListView1.Items.Clear()
'SQL STM
Dim sql As String = "SELECT * FROM Table1 "
cmd = New OleDbCommand(sql, con)
'OPEN CON, RETRIEVE, FILL LISTVIEW
Try
con.Open()
adapter = New OleDbDataAdapter(cmd)
adapter.Fill(dt)
'LOOP THROUGH DT
For Each row In dt.Rows
Populate(row(0), row(1)) 'Index of database row
Next
'CLEAR DATATABLE
dt.Rows.Clear()
con.Close()
Catch ex As Exception
MsgBox(ex.Message)
con.Close()
End Try
End Sub
Private Sub Populate(lname As String, aamount As String)
'ROW ARRAY
Dim row As String() = New String() {lname, aamount}
Dim item As ListViewItem = New ListViewItem(row)
'ADD TO ROWS COLLECTION
ListView1.Items.Add(item)
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Retrieve()
End Sub
Private Sub ListView1_MouseClick(sender As Object, e As MouseEventArgs) Handles ListView1.MouseClick
Dim llname As String = ListView1.SelectedItems(0).SubItems(0).Text
Dim amounts As String = ListView1.SelectedItems(0).SubItems(1).Text
TextBox1.Text = llname
TextBox2.Text = amounts
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim amounts As String = ListView1.SelectedItems(0).SubItems(0).Text
UpdateLV(amounts)
End Sub
Private Sub ClearBox()
TextBox1.Text = ""
TextBox2.Text = ""
End Sub
End Class

Answer 1

Mathematical operation should not be done using strings. This is a real basic principle that many VB.NET programmers don't think enough thanks to the forgiveness allowed by Option Strict Off in the VB.NET project settings.
If you are just starting a new project in VB.NET don't use this setting but switch it ASAP to On. This will give you an halt when you try to use strings as they were numbers and force you to do the appropriate conversion and checking on the values provided.
So your code that updates the amount rewritten
Private Sub UpdateLV(lname As String)
' Get the amount as a number (decimal for currency is the best)
Dim addAmt As Decimal
if Not decimal.TryParse(textbox2.Text, addAmt) Then
MessageBox.Show("Insert a valid amount please")
return
End If
' Sanity check
if addAmt <= 0 Then
MessageBox.Show("Amount should be > 0")
return
End If
'Updates last name and amount entered into the database
Dim sql As String = "UPDATE Table1 SET LastName=#name
,Amount=Amount+#amt
WHERE LastName=#oldname"
cmd = New OleDbCommand(sql, con)
Try
con.Open()
' Using an adapter here is wrong. You use directly the command
cmd.Parameters.Add("#name", OleDbType.VarWChar).Value = textBox1.Text
cmd.Parameters.Add("#amt", OleDbType.Decimal).Value = addAmt
cmd.Parameters.Add("#oldname", OleDbType.VarWChar).Value = lName
If (cmd.ExecuteNonQuery() > 0) Then
MsgBox("Successfully Updated")
End If
con.Close()
Retrieve()
ClearBox()
Catch ex As Exception
MsgBox(ex.Message)
con.Close()
End Try
End Sub
Something else is not clear in your code. What is the purpose of changing also the LastName here? Finally do not keep a global connection object. Instead create it when you need it and destroy it afterward with Using statement. It will be better for your memory footprint and for your database