How to reduce quantity of product in SQL

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private void btnConfirm_Click(object sender, EventArgs e)
        {
            string product_id = txtProductID.Text;
            string product_name = txtProductName.Text;
            string product_price = txtPrice.Text;
            string product_type = txtType.Text;
            string product_qty = txtQty.Text;
            string order_qty = txtOrderQty.Text;


            //2. create a sql insertion string
            string sql = "update productinfo set product_id = 'p1',product_name = 'p2',product_price = 'p3',product_type='p4',product_qty= (product_qty - order_qty),'p5'";
            sql += " where product_id ='" + product_id + "'";
            sql = sql.Replace("p1", product_id);
            sql = sql.Replace("p2", product_name);
            sql = sql.Replace("p3", product_price);
            sql = sql.Replace("p4", product_type);
            sql = sql.Replace("p5", product_qty);
            MessageBox.Show(sql);

            m1.ExecuteNonQuery(sql);
            MessageBox.Show("done");
            refreshProductList();
        }

where txtOrderqty.Text is the amount to be reduced in product_qty

What I have tried:

I tried

product_qty= (product_qty - order_qty),'p5'";

but it's not working

Posted 13-Sep-21 1:18am

Member 14961132

Updated 13-Sep-21 1:31am

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Richard Deeming · Answer 1 · 2021-09-13T01:31:00

Not like that!

Your code is vulnerable to SQL Injection[^]. NEVER use string concatenation to build a SQL query. ALWAYS use a parameterized query.

Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]

C#

const string sql = "update productinfo set product_name = @name, product_price = @price, product_type = @type, product_qty = product_qty - @order_qty where product_id = @id";

using (var connection = new SqlConnection("..."))
using (var command = new SqlCommand(sql, connection))
{
    command.Parameters.AddWithValue("@name", product_name);
    command.Parameters.AddWithValue("@price", product_price);
    command.Parameters.AddWithValue("@type", product_type);
    command.Parameters.AddWithValue("@order_qty", order_qty);
    command.Parameters.AddWithValue("@id", product_id);
    
    connection.Open();
    command.ExecuteNonQuery();
}