How Can MySQL's Depth-First Search Effectively Retrieve All Ancestors of a Node in a Hierarchical Structure?

Hierarchical Queries in MySQL: Traversing Ancestry with Depth-First Search

Determining ancestry relationships within hierarchical data is a common task in database management. In MySQL, hierarchical queries allow you to navigate and retrieve data from nested structures effectively.

Suppose you have a table named 'mytable' with two columns, 'a' and 'b,' representing a hierarchical parent-child relationship:

| a | b |
----------
| 1 | 2 |
| 2 | 3 |
| 3 | 4 |
| 4 | 5 |
| 3 | 6 |
| 4 | 7 |

Consider the scenario where you want to retrieve all the ancestors of a given node, for example, finding all the parents, grandparents, and so on, of node 5.

Solution utilizzando l'algoritmo di ricerca in profondità:

MySQL provides a hierarchical query solution using a depth-first search (DFS) approach. Here's a query that accomplishes this:

SELECT  @id :=
        (
        SELECT  senderid
        FROM    mytable
        WHERE   receiverid = @id
        ) AS person
FROM    (
        SELECT  @id := 5
        ) vars
STRAIGHT_JOIN
        mytable
WHERE   @id IS NOT NULL

Analysis:

• The nested query initializes the '@id' variable with the node (5) whose ancestors we want to find.
• The outer query iteratively retrieves the direct parent of the current '@id' using the 'senderid' field of 'mytable.'
• The 'STRAIGHT_JOIN' ensures that the query follows the hierarchy without any optimizations that could bypass the parent-child relationship.
• The '@id' variable is updated with the 'senderid' of the parent node in each iteration, and the process continues until there are no more parents.

By executing this query, you will retrieve a list of ancestors for node 5: 4, 3, 2, and 1. This method effectively traverses the hierarchy using a DFS approach, allowing you to navigate complex parent-child structures within your MySQL database.

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