What is the most efficient/elegant way to parse a flat table into a tree?

Now that MySQL 8.0 supports recursive queries, we can say that all popular SQL databases support recursive queries in standard syntax.

WITH RECURSIVE MyTree AS (    SELECT * FROM MyTable WHERE ParentId IS NULL    UNION ALL    SELECT m.* FROM MyTABLE AS m JOIN MyTree AS t ON m.ParentId = t.Id)SELECT * FROM MyTree;

I tested recursive queries in MySQL 8.0 in my presentation Recursive Query Throwdown in 2017.

Below is my original answer from 2008:

There are several ways to store tree-structured data in a relational database. What you show in your example uses two methods:

• Adjacency List (the "parent" column) and
• Path Enumeration (the dotted-numbers in your name column).

Another solution is called Nested Sets, and it can be stored in the same table too. Read "Trees and Hierarchies in SQL for Smarties" by Joe Celko for a lot more information on these designs.

I usually prefer a design called Closure Table (aka "Adjacency Relation") for storing tree-structured data. It requires another table, but then querying trees is pretty easy.

I cover Closure Table in my presentation Models for Hierarchical Data with SQL and PHP and in my book SQL Antipatterns: Avoiding the Pitfalls of Database Programming.

CREATE TABLE ClosureTable (  ancestor_id   INT NOT NULL REFERENCES FlatTable(id),  descendant_id INT NOT NULL REFERENCES FlatTable(id),  PRIMARY KEY (ancestor_id, descendant_id));

Store all paths in the Closure Table, where there is a direct ancestry from one node to another. Include a row for each node to reference itself. For example, using the data set you showed in your question:

INSERT INTO ClosureTable (ancestor_id, descendant_id) VALUES  (1,1), (1,2), (1,4), (1,6),  (2,2), (2,4),  (3,3), (3,5),  (4,4),  (5,5),  (6,6);

Now you can get a tree starting at node 1 like this:

SELECT f.* FROM FlatTable f   JOIN ClosureTable a ON (f.id = a.descendant_id)WHERE a.ancestor_id = 1;

The output (in MySQL client) looks like the following:

+----+| id |+----+|  1 | |  2 | |  4 | |  6 | +----+

In other words, nodes 3 and 5 are excluded, because they're part of a separate hierarchy, not descending from node 1.

Re: comment from e-satis about immediate children (or immediate parent). You can add a "path_length" column to the ClosureTable to make it easier to query specifically for an immediate child or parent (or any other distance).

INSERT INTO ClosureTable (ancestor_id, descendant_id, path_length) VALUES  (1,1,0), (1,2,1), (1,4,2), (1,6,1),  (2,2,0), (2,4,1),  (3,3,0), (3,5,1),  (4,4,0),  (5,5,0),  (6,6,0);

Then you can add a term in your search for querying the immediate children of a given node. These are descendants whose path_length is 1.

SELECT f.* FROM FlatTable f   JOIN ClosureTable a ON (f.id = a.descendant_id)WHERE a.ancestor_id = 1  AND path_length = 1;+----+| id |+----+|  2 | |  6 | +----+

Re comment from @ashraf: "How about sorting the whole tree [by name]?"

Here's an example query to return all nodes that are descendants of node 1, join them to the FlatTable that contains other node attributes such as name, and sort by the name.

SELECT f.nameFROM FlatTable f JOIN ClosureTable a ON (f.id = a.descendant_id)WHERE a.ancestor_id = 1ORDER BY f.name;

Re comment from @Nate:

SELECT f.name, GROUP_CONCAT(b.ancestor_id order by b.path_length desc) AS breadcrumbsFROM FlatTable f JOIN ClosureTable a ON (f.id = a.descendant_id) JOIN ClosureTable b ON (b.descendant_id = a.descendant_id) WHERE a.ancestor_id = 1 GROUP BY a.descendant_id ORDER BY f.name+------------+-------------+| name       | breadcrumbs |+------------+-------------+| Node 1     | 1           || Node 1.1   | 1,2         || Node 1.1.1 | 1,2,4       || Node 1.2   | 1,6         |+------------+-------------+

A user suggested an edit today. SO moderators approved the edit, but I am reversing it.

The edit suggested that the ORDER BY in the last query above should be ORDER BY b.path_length, f.name, presumably to make sure the ordering matches the hierarchy. But this doesn't work, because it would order "Node 1.1.1" after "Node 1.2".

If you want the ordering to match the hierarchy in a sensible way, that is possible, but not simply by ordering by the path length. For example, see my answer to MySQL Closure Table hierarchical database - How to pull information out in the correct order.

If you use nested sets (sometimes referred to as Modified Pre-order Tree Traversal) you can extract the entire tree structure or any subtree within it in tree order with a single query, at the cost of inserts being more expensive, as you need to manage columns which describe an in-order path through thee tree structure.

For django-mptt, I used a structure like this:

id  parent_id  tree_id  level  lft  rght--  ---------  -------  -----  ---  ---- 1       null        1      0    1    14 2          1        1      1    2     7 3          2        1      2    3     4 4          2        1      2    5     6 5          1        1      1    8    13 6          5        1      2    9    10 7          5        1      2    11   12

Which describes a tree which looks like this (with id representing each item):

 1 +-- 2 |   +-- 3 |   +-- 4 | +-- 5     +-- 6     +-- 7

Or, as a nested set diagram which makes it more obvious how the lft and rght values work:

 __________________________________________________________________________|  Root 1                                                                  ||   ________________________________    ________________________________   ||  |  Child 1.1                     |  |  Child 1.2                     |  ||  |   ___________    ___________   |  |   ___________    ___________   |  ||  |  |  C 1.1.1  |  |  C 1.1.2  |  |  |  |  C 1.2.1  |  |  C 1.2.2  |  |  |1  2  3___________4  5___________6  7  8  9___________10 11__________12 13 14|  |________________________________|  |________________________________|  ||__________________________________________________________________________|

As you can see, to get the entire subtree for a given node, in tree order, you simply have to select all rows which have lft and rght values between its lft and rght values. It's also simple to retrieve the tree of ancestors for a given node.

The level column is a bit of denormalisation for convenience more than anything and the tree_id column allows you to restart the lft and rght numbering for each top-level node, which reduces the number of columns affected by inserts, moves and deletions, as the lft and rght columns have to be adjusted accordingly when these operations take place in order to create or close gaps. I made some development notes at the time when I was trying to wrap my head around the queries required for each operation.

In terms of actually working with this data to display a tree, I created a tree_item_iterator utility function which, for each node, should give you sufficient information to generate whatever kind of display you want.

More info about MPTT:

• Trees in SQL
• Storing Hierarchical Data in a Database
• Managing Hierarchical Data in MySQL

It's a quite old question, but as it's got many views I think it's worth to present an alternative, and in my opinion very elegant, solution.

In order to read a tree structure you can use recursive Common Table Expressions (CTEs). It gives a possibility to fetch whole tree structure at once, have the information about the level of the node, its parent node and order within children of the parent node.

Let me show you how this would work in PostgreSQL 9.1.

1. Create a structure

   CREATE TABLE tree (    id int  NOT NULL,    name varchar(32)  NOT NULL,    parent_id int  NULL,    node_order int  NOT NULL,    CONSTRAINT tree_pk PRIMARY KEY (id),    CONSTRAINT tree_tree_fk FOREIGN KEY (parent_id)       REFERENCES tree (id) NOT DEFERRABLE);insert into tree values  (0, 'ROOT', NULL, 0),  (1, 'Node 1', 0, 10),  (2, 'Node 1.1', 1, 10),  (3, 'Node 2', 0, 20),  (4, 'Node 1.1.1', 2, 10),  (5, 'Node 2.1', 3, 10),  (6, 'Node 1.2', 1, 20);

2. Write a query

   WITH RECURSIVE tree_search (id, name, level, parent_id, node_order) AS (  SELECT     id,     name,    0,    parent_id,     1   FROM tree  WHERE parent_id is NULL  UNION ALL   SELECT     t.id,     t.name,    ts.level + 1,     ts.id,     t.node_order   FROM tree t, tree_search ts   WHERE t.parent_id = ts.id ) SELECT * FROM tree_search WHERE level > 0 ORDER BY level, parent_id, node_order;

   Here are the results:

    id |    name    | level | parent_id | node_order ----+------------+-------+-----------+------------  1 | Node 1     |     1 |         0 |         10  3 | Node 2     |     1 |         0 |         20  2 | Node 1.1   |     2 |         1 |         10  6 | Node 1.2   |     2 |         1 |         20  5 | Node 2.1   |     2 |         3 |         10  4 | Node 1.1.1 |     3 |         2 |         10(6 rows)

   The tree nodes are ordered by a level of depth. In the final output we would present them in the subsequent lines.

   For each level, they are ordered by parent_id and node_order within the parent. This tells us how to present them in the output - link node to the parent in this order.

   Having such a structure it wouldn't be difficult to make a really nice presentation in HTML.

   Recursive CTEs are available in PostgreSQL, IBM DB2, MS SQL Server and Oracle.

   If you'd like to read more on recursive SQL queries, you can either check the documentation of your favourite DBMS or read my two articles covering this topic:

   • Do It In SQL: Recursive Tree Traversal
   • Get to know the power of SQL recursive queries