ปัญหาใช้คำสั่งSQL UPDATEบวกเพิ่มสินค้าแต่ในฐานข้อมูลบวกเพิ่มไปอีก1 แต่ในฐานข้อมูลมันบวกให้อีก1

    Dim a as integer
    Dim sqlText as string

    a = cint(Textbox1.text)
    a+=1

    sqlText = "Update Product SET ProQuantity = " & a.ToString() & "where ProID =' " & Request.QueryString("ProID") & " ' "

     int a = 0;
     string sqlText = "";

     a = (int)Textbox1.text;
     a+=1;

     sqlText = "Update Product SET ProQuantity = " & a.ToString() & "where ProID =' " & Request.QueryString("ProID") & " ' ";