如何使用 SQL Server 作为数据库在 Visual Studio C# 中创建销售按钮代码

Question

I'm using Visual Studio C# and I'm having a conflict on how to create a sell button as well how can I get the total amount of product that had been sold here is my code

try
{
    if (txtID.Text != "" && txtCategory.Text != "" && 
        txtName.Text != "" && txtQty.Text != "" && 
        txtQty.Text != "" && PriceText.Text != "" && 
        SuppNameText.Text != "")
    {
        con.Open();
        SqlCommand cmd = new SqlCommand(@"INSERT INTO Sold_Inventory 
            (ProductName, Description, Category, Quantity_Sold, Price,
             TotalAmount, SuppliersName) 
             VALUES(@ProductName, @Description, @Category, @Quantity, @Price,
             @Supplier)
             WHERE TotalAmount = @Quantity * @Price", con);

        cmd.Parameters.AddWithValue("@ProductName", txtID.Text);
        cmd.Parameters.AddWithValue("@Description", txtName.Text);
        cmd.Parameters.AddWithValue("@Category", txtCategory.Text);
        cmd.Parameters.AddWithValue("@Quantity", txtQty.Text);
        cmd.Parameters.AddWithValue("@Price", PriceText.Text);
        cmd.Parameters.AddWithValue("@Supplier", SuppNameText.Text);

        cmd.ExecuteNonQuery();

        SqlCommand cmd1 = new SqlCommand(@"UPDATE Quantity 
               FROM Inventory SET Quantity = Quantity - @Quantity");
        cmd1.Parameters.AddWithValue("@Quantity", txtQty.Text);

        cmd1.ExecuteNonQuery();
        con.Close();

        ClearTextbox();

        btnSearch.Enabled = true;
        txtSearch.Enabled = true;
        groupBox4.Enabled = true;
        btnSubmit.Enabled = false;
        btnCancel.Enabled = false;
        ClearTextbox();
        DisabledTextbox();
        btnAdd.Enabled = true;

        RefreshDGV(this.dataGridView1);
    }
    else
    {
        MessageBox.Show("You left an empty field!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

Answer 1

The syntax of the first SqlCommand is wrong.
INSERT INTO doesn't require a WHERE clause and you are missing one parameter placeholder in your query text.

So the first fix is around your command text

SqlCommand cmd = new SqlCommand(@"INSERT INTO Sold_Inventory 
     (ProductName,Description,Category,Quantity_Sold,Price,
      TotalAmount,SuppliersName) 
      VALUES(@ProductName,@Description,@Category,@Quantity,@Price,
      @TotalAmount, @Supplier)", con);

Now calculate the value for the @TotalAmount parameter in code like this....

decimal total = Convert.ToDecimal(txtQty.Text) * 
                Convert.ToDecimal(PriceText.Text);
cmd.Parameters.AddWithValue("@TotalAmount", total);

Finally, the command that decrements the quantity should identify the exact record for the product that you have sold. Here the WHERE clause is fundamental.

I suppose that your Product table has a record for the product that you have sold identified by something like a ProductID primary key. (Also you need to add the connection when you build the command)

SqlCommand cmd1 = new SqlCommand(@"Update Inventory 
           SET Quantity = Quantity - @Quantity
           WHERE ProductID = @productID", con);

There are other considerations to your code. First, forget the AddWithValue method. It is an easy shortcut, but it can create a lot of problems if you pass a string as parameter value and the column that receives the value is not a text field (the automatic conversion from string to decimal could fail because the localization of your client code doesn't match the rules used by the database engine)

Better use always the Add method specifying the datatype of the parameter.
For example, the Price column probably wants a decimal not a string, so

decimal price = Convert.ToDecimal(PriceText.Text);
cmd.Parameters.Add("@Price", SqlDbType.Decimal).Value = price;

Of course the same applies to all other values. Don't let AddWithValue figure out what do you mean with your parameter. Be explicit.