Executing sql when select a different option

Hello people,
I have a lot of sqls, now I want one sql to be executed  depending on  what has been selected in a combo  box or drop down menu.It should display data from the database. problem  is that  it  is not executing the sql when I selected a different Item.I have put AutoPostback True.
 For example
private void Dropdown1_SelectedIndexChanged(object sender, System.EventArgs e)
        {
        
                if  
if (A==1)
{
sql="";

}

else if (A==2)
{

sql="";
}
else if (A==3)
{
sql="";
}
        }



These are not working. How can we achieve this??

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