Creating a list tree with SQLite

Question

I'm trying to make a hierarchical list with PHP and an SQLite table setup like this:

    |   itemid  |   parentid    |   name    |
    -----------------------------------------
    |   1       |   null        |   Item1   |
    |   2       |   null        |   Item2   |
    |   3       |   1           |   Item3   |
    |   4       |   1           |   Item4   |
    |   5       |   2           |   Item5   |
    |   6       |   5           |   Item6   |

The lists would be built with unordered lists and allow for this type of tree structure:

Item1
    |_Item3
    |_Item4
Item2
    |_Item5
        |_Item6

I've seen this done with directories and flat arrays, but I can't seem to make it work right with this structure and without a depth limit.

Answer 1

You're using a textbook design for storing hierarchical data in an SQL database. This design is called Adjacency List, i.e. each node in the hierarchy has a parentid foreign key to its immediate parent.

With this design, you can't generate a tree like you describe and support arbitrary depth for the tree. You've already figured this out.

Most other SQL databases (PostgreSQL, Microsoft, Oracle, IBM DB2) support recursive queries, which solve this problem. But SQLite and MySQL don't support this feature of SQL yet.

So you need another solution to store the hierarchy. There are several solutions for this. See my presentation Models for Hierarchical Data with PHP and MySQL for descriptions and examples.

I usually prefer a design I call Closure Table, but each design has strength and weaknesses. Which one is best for your project depends on what kinds of queries you need to do efficiently with your data. So you should go study the solutions and choose one for yourself.

Answer 2

I know this was asked log time ago, but with current SQLite version it is trivial to do and no need level depth as @Bill-Karwin says. So the correct answer should be reconsidered :)

My table has columns MCTMPLID and REF_TMPLID and my structure starting node is called ROOT

CREATE TABLE MyStruct (
  `TMPLID` text,
  `REF_TMPLID` text
);

INSERT INTO MyStruct
  (`TMPLID`, `REF_TMPLID`)
VALUES
  ('Root', NULL),
  ('Item1', 'Root'),
  ('Item2', 'Root'),
  ('Item3', 'Item1'),
  ('Item4', 'Item1'),
  ('Item5', 'Item2'),
  ('Item6', 'Item5');

And here is the main query, that builds tree structure

WITH RECURSIVE
  under_root(name,level) AS (
    VALUES('Root',0)
    UNION ALL
    SELECT tmpl.TMPLID, under_root.level+1
      FROM MyStruct as tmpl JOIN under_root ON tmpl.REF_TMPLID=under_root.name
     ORDER BY 2 DESC
  )
SELECT substr('....................',1,level*3) || name as TreeStructure FROM under_root

And here is result

Root
...Item1
......Item3
......Item4
...Item2
......Item5
.........Item6

I'm sure this can be modified to work tik OP's table structure, so let this sample be starting point Documentation and some samples https://www.sqlite.org/lang_with.html#rcex1