I've made a little example. I've test it and it works fine. 
Note that I've made a class MyDataBase to simulate your database:
public void CreateTreeView()
{
    TreeView myTreeview = new TreeView();
    myTreeview.Dock = DockStyle.Fill;
    this.Controls.Add(myTreeview);
    foreach (string field in MyDataBase.FieldsInMyColumn())
    {
        string[] elements = field.Split('/');
        TreeNode parentNode = null;
        for (int i = 0; i < elements.Length - 1; ++i)
        {                                        
            if (parentNode == null)
            {
                bool exits = false;
                foreach (TreeNode node in myTreeview.Nodes)
                {
                    if (node.Text == elements[i])
                    {
                        exits = true;
                        parentNode = node;   
                    }
                }
                if (!exits)
                {
                    TreeNode childNode = new TreeNode(elements[i]);
                    myTreeview.Nodes.Add(childNode);
                    parentNode = childNode;
                }
            }
            else
            {
                bool exits = false;
                foreach (TreeNode node in parentNode.Nodes)
                {
                    if (node.Text == elements[i])
                    {
                        exits = true;
                        parentNode = node;
                    }
                }
                if (!exits)
                {
                    TreeNode childNode = new TreeNode(elements[i]);
                    parentNode.Nodes.Add(childNode);
                    parentNode = childNode;
                }
            }
        }
        if (parentNode != null)
        {
           parentNode.Nodes.Add(elements[elements.Length - 1]);
        }
    }         
}
EDIT
Here I paste my auxiliar code, that you don´t need, but it will help you to understand my code, or to copy/paste and try it by yourself.
public static class MyDataBase
{
    private static List<string> fields = new List<string>();
    public static void AddField(string field)
    {
        fields.Add(field);
    }
    public static IList<string> FieldsInMyColumn()
    {
        return fields;
    }
}
Constructor in form1
public Form1()
{
    InitializeComponent();
    MyDataBase.AddField("jsmith/project1/hello.cs");
    MyDataBase.AddField("jsmith/project1/what.cs");
    MyDataBase.AddField("jsmith/project2/hello.cs");
    CreateTreeView();
}