Question

How would you persist root nodes of custom tree structures in such a way that they could be retrieved with a simple SELECT ?

Pseudo-code

class Node {
    Long id;
    String label;
    Node parent;
    List<Node> children;
}

Edit: Tables could be as follows. Feel free to suggest improvement.

CREATE TABLE NODES(
    ID INTEGER NOT NULL UNIQUE,
    LABEL VARCHAR(255)
);

CREATE TABLE PARENTS_CHILDREN(
    PARENT_ID INTEGER NOT NULL,
    CHILD_ID INTEGER NOT NULL
);

Answer 1

You could use an adjacency list:

node   parent
1      NULL
2      1
3      1
4      2
5      NULL
6      5

The root nodes can be found with WHERE parent IS NULL.