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();