How to represent a data tree in SQL?

Question

I\'m writing a data tree structure that is combined from a Tree and a TreeNode. Tree will contain the root and the top level actions on the data. I\'m using a UI library to

Answer 1

If you are using PostgreSQL you can use ltree, a package in the contrib extension (comes by default) which implements the tree data structure.

From the docs:

CREATE TABLE test (path ltree);
INSERT INTO test VALUES ('Top');
INSERT INTO test VALUES ('Top.Science');
INSERT INTO test VALUES ('Top.Science.Astronomy');
INSERT INTO test VALUES ('Top.Science.Astronomy.Astrophysics');
INSERT INTO test VALUES ('Top.Science.Astronomy.Cosmology');
INSERT INTO test VALUES ('Top.Hobbies');
INSERT INTO test VALUES ('Top.Hobbies.Amateurs_Astronomy');
INSERT INTO test VALUES ('Top.Collections');
INSERT INTO test VALUES ('Top.Collections.Pictures');
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy');
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Stars');
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Galaxies');
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Astronauts');
CREATE INDEX path_gist_idx ON test USING GIST (path);
CREATE INDEX path_idx ON test USING BTREE (path);

You can do queries like:

ltreetest=> SELECT path FROM test WHERE path <@ 'Top.Science';
                path
------------------------------------
 Top.Science
 Top.Science.Astronomy
 Top.Science.Astronomy.Astrophysics
 Top.Science.Astronomy.Cosmology
(4 rows)

Answer 2

It depends on how you will be querying and updating the data. If you store all the data in one row, it's basically a single unit that you can't query into or partially update without rewriting all the data.

If you want to store each element as a row, you should first read Managing Hierarchical Data in MySQL (MySQL specific, but the advice holds for many other databases too).

If you're only ever accessing an entire tree, the adjacency list model makes it difficult to retrieve all nodes under the root without using a recursive query. If you add an extra column that links back to the head then you can do SELECT * WHERE head_id = @id and get the whole tree in one non-recursive query, but it denormalizes the database.

Some databases have custom extensions that make storing and retrieving heirarchical data easier, for example Oracle has CONNECT BY.