怎样根据数据库表中的字段生成如上树形结构

var conn = new SqlConnection("......");conn.Open();var cmd = conn.CreateCommand();cmd.CommandText = "select 电机编号, 电机类别, 电机型号, 试验日期 from 表";var reader = cmd.ExecuteReader();var rootNode = this.treeView1.Nodes.Add("车用电机NVH试验数据");Func<TreeNode, string, TreeNode> func = (v1, v2) =>{    TreeNode[] nodes = v1.Nodes.Find(v2, false);    if (nodes.Length > 0)    {        return nodes[0];    }    TreeNode n = v1.Nodes.Add(v2);    n.Name = v2;    return n;};while (reader.Read()){    var num = reader.GetString(0);    var category = reader.GetString(1);    var model = reader.GetString(2);    var year = reader.GetDateTime(3).ToString("yyyy") + "年";    var cateNode = func(rootNode, category);    var modelNode = func(cateNode, model);    var yearNode = func(modelNode, year);    yearNode.Nodes.Add(num);}reader.Dispose();cmd.Dispose();conn.Dispose();