How do I subtract the quantity of my products listed in listbox directly to my database of products? (Mysql)

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C#

MySQL

ListBox

First of all, I am not really into hard coding and such, I really did try to search for a solution to my problem but I am not getting some of the explanation. Or maybe it is not the way I code mine so I cannot understand.

Technically I have 3 lisboxes for the quantity, product name, and price of the product. When a product is added on a cart, it automatically lists down in the listbox.

I am trying to subtract the quantity of the product stocks saved in the database, to the number of products listed on the cart.

My code works when I only buy one product, but when I am trying to buy 2 or more products, nothing gets subtracted to the database of my products.

PS. Sorry if ever I incorrectly posted my question, it is just my first time posting here. Thank you in advance!

What I have tried:

string item = string.Empty;
int quantity = 0;

for (int i = 0; i <= LBItemName.Items.Count - 1; i++)
{
    item += Convert.ToString(LBItemName.Items[i]);
}

for (int j = 0; j <= LBItemQuantity.Items.Count - 1; j++)
{
    quantity += Convert.ToInt32(LBItemQuantity.Items[j]);
}

MySqlCommand tryCommand = new MySqlCommand("UPDATE QuizonVet.Product SET
    numberofstocks = (numberofstocks - '" + quantity + "') where productname='" + 
    quantity + "';", myConnection);

myReader = tryCommand.ExecuteReader();
myReader.Close();