How to Query Hierarchical Data in MySQL Without CONNECT BY PRIOR?

Querying Hierarchical Data in MySQL without "Connect By Prior"

In MySQL, despite the absence of an explicit "Connect By Prior" clause, it is possible to retrieve hierarchical data using a combination of recursive techniques.

Recursive Traversal Algorithm

To recursively traverse a hierarchical table like tb_Tree, follow these steps:

1. Select rows where ParentId matches the desired root node.
2. Collect the Id values of the selected rows.
3. Repeat steps 1 and 2 for each Id in the collected list.

This recursive process continues until all leaf nodes are identified.

Depth-Based Approach

If you know the maximum depth of the tree, you can join the table to itself repeatedly to reach the deepest level and then filter out any remaining NULL values.

Nested Set Representation

Alternatively, you could modify the table structure to use a nested set representation. This involves adding additional columns to represent the left and right bounds of each node in the hierarchy.

Example Query for Retrieving Children

To retrieve all children of a specific node with Id equal to X, you would use the following query:

SELECT * FROM tb_Tree WHERE ParentId IN (
  SELECT Id FROM tb_Tree WHERE ParentId = X
  UNION ALL
  /* Recursive traversal of children */
  SELECT Id FROM tb_Tree WHERE Id IN (
    SELECT Id FROM tb_Tree WHERE ParentId = X
  )
);

This query recursively traverses the hierarchy, collecting all Id values associated with child nodes.

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