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");
TreeNode[] nodes;
Func<TreeNode, string, TreeNode> func = (v1, v2) => (nodes = v1.Nodes.Find(v2, false)).Length > 0 ? nodes[0] : v1.Nodes.Add(v2);

while (reader.Read())
{
    var num = reader.GetString(0);
    var category = reader.GetString(1);
    var model = reader.GetString(2);
    var date = reader.GetString(3);
    var year = date.Substring(0, 4) + "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();