Querying All Parents in a MySQL Hierarchy with More Than One Parent per Node

When dealing with hierarchical data, querying all parents of a node can be straightforward when every node has only one parent. However, things become more complex when nodes have more than one parent. In this article, we’ll explore the challenges and solutions for querying all parents in a MySQL hierarchy where nodes can have multiple parents.

Understanding Hierarchical Data in MySQL

To approach this problem, it’s essential to understand how hierarchical data is stored in MySQL. By default, MySQL doesn’t support recursive queries out of the box. However, since version 5.7, MySQL supports recursive Common Table Expressions (CTEs), which allow us to query hierarchical data using standard SQL.

For this article, we’ll focus on versions prior to 5.7 and explore alternative approaches to querying all parents in a hierarchy where nodes have more than one parent.

The Problem with Multiple Parents

Let’s examine the given table structure and how it affects our queries:

+----+---------+---------+
| id | node_id | parent_id |
+----+---------+---------+
| 1  |       1 |       2  |
| 2  |       2 |       3  |
| 3  |       2 |       4  |
| 4  |       4 |       5  |
| 5  |       5 |       6  |
| 6  |       6 |       7  |
+----+---------+---------+

In this example, node 2 has two parents: 3 and 4. If we query the parent IDs of node 1, we expect to get all parent nodes (3, 4, 5, 6, and 7). However, using a traditional recursive approach would only return 3, 4, and 2.

Traditional Recursive Approach

A common method for querying hierarchical data is by using a recursive CTE. In MySQL 5.7 and later, we can use the following query:

WITH RECURSIVE parent_hierarchy AS (
    SELECT id, node_id, parent_id, 0 as level
    FROM nodes
    WHERE parent_id IS NULL -- anchor query for root nodes

    UNION ALL

    SELECT n.id, n.node_id, n.parent_id, ph.level + 1
    FROM nodes n
    JOIN parent_hierarchy ph ON n.parent_id = ph.id
)
SELECT id, node_id, parent_id
FROM parent_hierarchy;

This query uses a recursive CTE to traverse the hierarchy. The anchor query selects the root nodes (nodes with no parent), and the recursive step joins each child node with its parent’s parent hierarchy.

However, this approach has limitations when dealing with nodes that have multiple parents. In such cases, MySQL will only return one parent ID per level of recursion. To illustrate this, let’s modify our table to include a new node 8 with multiple parents:

+----+---------+---------+
| id | node_id | parent_id |
+----+---------+---------+
| 1  |       1 |       2  |
| 2  |       2 |       3  |
| 3  |       2 |       4  |
| 4  |       4 |       5  |
| 5  |       5 |       6  |
| 6  |       6 |       7  |
| 7  |       7 |       8  |
| 8  |       8 |       9  |
+----+---------+---------+

In this updated table, node 2 still has two parents (3 and 4), but we’ve added a new node 8 with multiple parents. If we run the recursive query on node 1, it will only return 3 as the parent ID.

Alternative Approach: Using FIND_IN_SET

Another approach to querying all parents in a hierarchy where nodes have more than one parent is by using FIND_IN_SET. This method relies on indexing and can be more efficient for large datasets:

SELECT DISTINCT n1.id AS parent_id, n2.id AS child_id
FROM nodes n1
JOIN nodes n2 ON FIND_IN_SET(n2.parent_id, FIND_IN_SETセット('parents', n1.node_id))
WHERE n1.parent_id IS NOT NULL;

In this query, we use the FIND_IN_SET function to find all parent IDs associated with a given node ID. We then join this result with the nodes table to get the corresponding child nodes.

To create an index on parents, you can run the following command:

CREATE INDEX idx_parents ON nodes (node_id, FIND_IN_SET(parent_id, FIND_IN_SET_SET('parents', node_id)));

This index will improve query performance for large datasets.

Conclusion

Querying all parents in a MySQL hierarchy where nodes have more than one parent can be challenging. By understanding the limitations of traditional recursive approaches and leveraging alternative methods like FIND_IN_SET, you can efficiently retrieve all parent nodes for a given node ID.

While this article has focused on solutions for MySQL versions prior to 5.7, keep in mind that MySQL 5.7 introduced recursive CTEs, which provide a more elegant solution for querying hierarchical data.

Last modified on 2023-08-02