itemp=mydb.exeSqlForDataSet("select * from user where username='"+username.Text+"'").Tables[0].Rows.Count;
//if the row's count isn't 0
if(itemp>0)
{
Response.Write("<script language = javascript>alert('此用户已经存在!');</script>");
}
public DataSet exeSqlForDataSet(string QueryString)
{
SqlCommand cmd=new SqlCommand();
cmd.Connection=this.conn ;
if(inTransaction)
cmd.Transaction=trans;
DataSet ds = new DataSet();
SqlDataAdapter ad = new SqlDataAdapter();
cmd.CommandText=QueryString;
ad.SelectCommand =cmd;
ad.Fill(ds);
return ds;
}