Check if stock exists then update else insert in database in ASP.Net using C# and VB.Net

Hi Micah, 

  Please refer below Sample.

 HTML

<table>
    <tr>
        <th>Item</th>
        <td>
            <asp:TextBox runat="server" ID="txtItem" /></td>
    </tr>
    <tr>
        <th>Qty</th>
        <td><asp:TextBox runat="server" ID="txtQty" /></td>
    </tr>
    <tr>
        <td >
            <asp:Button Text="Update" runat="server" ID="btnUpdate" OnClick="OnUpdate" /></td>
    </tr>
</table>
<hr />
<asp:GridView runat="server" ID="GvStockUpdate" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="Item" HeaderText="Item" />
        <asp:BoundField DataField="Qty" HeaderText="Qty" />
    </Columns>
</asp:GridView>

Namespaces

C#

using System.Data;
using System.Data.SqlClient;
using System.Configuration;

VB.Net

Imports System.Data
Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid();
    }
}

private void BindGrid()
{
    string conn = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conn))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT Item, Qty FROM StockUpdate", con))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    GvStockUpdate.DataSource = dt;
                    GvStockUpdate.DataBind();
                }
            }
        }
    }
}

protected void OnUpdate(object sender, EventArgs e)
{
    string availableQuantity = "";
    string conn = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conn))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT Qty FROM StockUpdate WHERE Item = @item", con))
        {
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@item", txtItem.Text);
            con.Open();
            availableQuantity = Convert.ToString(cmd.ExecuteScalar());
            con.Close();
        }
    }

    if (string.IsNullOrEmpty(availableQuantity))
    {
        using (SqlConnection con = new SqlConnection(conn))
        {
            using (SqlCommand cmd = new SqlCommand("INSER INTO StockUpdate (Item, Qty) VALUES (@item, @Qty)", con))
            {
                cmd.Parameters.AddWithValue("@item", txtItem.Text);
                cmd.Parameters.AddWithValue("@Qty", txtQty.Text);
                con.Open();
                availableQuantity = Convert.ToString(cmd.ExecuteScalar());
            }
        }
    }
    else
    {
        int qty = Convert.ToInt32(availableQuantity) + Convert.ToInt32(txtQty.Text);
        using (SqlConnection con = new SqlConnection(conn))
        {
            using (SqlCommand cmd = new SqlCommand("UPDATE StockUpdate SET Qty = @Qty WHERE Item = @Item", con))
            {
                using (SqlDataAdapter sda = new SqlDataAdapter())
                {
                    cmd.Parameters.AddWithValue("@Item", txtItem.Text);
                    cmd.Parameters.AddWithValue("@Qty", qty);
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
            }
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindGrid()
    End If
End Sub

Private Sub BindGrid()
    Dim conn As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(conn)
        Using cmd As SqlCommand = New SqlCommand("SELECT Item, Qty FROM StockUpdate", con)
            Using sda As SqlDataAdapter = New SqlDataAdapter(cmd)
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    GvStockUpdate.DataSource = dt
                    GvStockUpdate.DataBind()
                End Using
            End Using
        End Using
    End Using
End Sub

Protected Sub OnUpdate(ByVal sender As Object, ByVal e As EventArgs)
    Dim availableQuantity As String = ""
    Dim conn As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(conn)
        Using cmd As SqlCommand = New SqlCommand("SELECT Qty FROM StockUpdate WHERE Item = @item", con)
            cmd.Connection = con
            cmd.Parameters.AddWithValue("@item", txtItem.Text)
            con.Open()
            availableQuantity = Convert.ToString(cmd.ExecuteScalar())
            con.Close()
        End Using
    End Using

    If String.IsNullOrEmpty(availableQuantity) Then
        Using con As SqlConnection = New SqlConnection(conn)
            Using cmd As SqlCommand = New SqlCommand("INSER INTO StockUpdate (Item, Qty) VALUES (@item, @Qty)", con)
                cmd.Parameters.AddWithValue("@item", txtItem.Text)
                cmd.Parameters.AddWithValue("@Qty", txtQty.Text)
                con.Open()
                availableQuantity = Convert.ToString(cmd.ExecuteScalar())
            End Using
        End Using
    Else
        Dim qty As Integer = Convert.ToInt32(availableQuantity) + Convert.ToInt32(txtQty.Text)
        Using con As SqlConnection = New SqlConnection(conn)
            Using cmd As SqlCommand = New SqlCommand("UPDATE StockUpdate SET Qty = @Qty WHERE Item = @Item", con)
                Using sda As SqlDataAdapter = New SqlDataAdapter()
                    cmd.Parameters.AddWithValue("@Item", txtItem.Text)
                    cmd.Parameters.AddWithValue("@Qty", qty)
                    con.Open()
                    cmd.ExecuteNonQuery()
                    con.Close()
                End Using
            End Using
        End Using
    End If
End Sub

Screenshot

SQL