帮我看看select语句，我应怎样写才对？
CS:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings[ "SQLConnString1 "].ConnectionString);

string strsql = "select * from Item where 1=1 ";

SqlCommand myCommand = new SqlCommand(strsql, conn);


string Pid = Request.QueryString[ "Pid "];

if (Pid != null)
{

myCommand.Parameters.Add(new SqlParameter( "@Pid ", SqlDbType.VarChar, 50));

myCommand.Parameters[ "@Pid "].Value = Pid;
strsql += " and ProductId=@Pid ";　//参数不起作用。应怎么写才对呢？
}


conn.Open();

SqlDataReader myReader = myCommand.ExecuteReader();
DataList1.DataSource = myReader;
DataList1.DataBind();
conn.Close();



}
}

[解决办法]
CS:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings[ "SQLConnString1 "].ConnectionString);

string strsql = "select * from Item where 1=1 ";

SqlCommand myCommand = null;


string Pid = Request.QueryString[ "Pid "];

if (Pid != null)
{
strsql += " and ProductId=@Pid ";　
myCommand = new SqlCommand(strsql, conn);
myCommand.Parameters.Add(new SqlParameter( "@Pid ",SqlDbType.VarChar, 50));

myCommand.Parameters[ "@Pid "].Value = Pid;
//参数不起作用。应怎么写才对呢？
}else
{
myCommand= new SqlCommand(strsql, conn);
}


conn.Open();

SqlDataReader myReader = myCommand.ExecuteReader();
DataList1.DataSource = myReader;
DataList1.DataBind();
conn.Close();



}
}

[解决办法]
if (Pid != null)
{
myCommand.CommandText =myCommand.CommandText + " and ProductId=@Pid "
myCommand.Parameters.Add(new SqlParameter( "@Pid ", SqlDbType.VarChar, 50));
myCommand.Parameters[ "@Pid "].Value = Pid;

}