Retrieve top-level parent MySQL

I have the following table:

  id  |  parent_id  |  searchable  |  value
--------------------------------------------
  1   |      0      |      0       |    a
  2   |      1      |      0       |    b
  3   |      2      |      1       |    c
  4   |      0      |      0       |    d
  5   |      4      |      1       |    e
  6   |      0      |      0       |    f
  7   |      6      |      0       |    g
  8   |      6      |      0       |    h
  9   |      0      |      1       |    i

I need to extract all the top level records (so the ones where the parent_id = 0). But only the records where the parent OR one of his children is searchable (searchable = 1)

So in this case, the output should be:

  id  |  parent_id  |  searchable  |  value
--------------------------------------------
  1   |     0       |      0       |    a
  4   |     0       |      0       |    d
  9   |     0       |      1       |    i

Because these are all top-level records and it self or one of his childeren (doesn't matter how 'deep' the searchable child is) is searchable.

I am working with MySQL. I am not really sure if it is possible to write this with just one query, but I assume it should be done with a piece of recursive code or a function.

** Note: it is unknown how 'deep' the tree goes.

Solution

You will have to use stored procedure to do it.

Find all rows with searchable = 1, store their ids and parent_ids in a temp table. Then do self-joins to add parents to this temp table. Repeat until no more rows can be added (obviously better make sure tree is not cyclic). At the end you have a table only with rows that have a searchable descendant somewhere down the tree, so just show only rows with no parent (at the top).

Assuming your table is called 'my_table' this one should work:

DELIMITER //
DROP PROCEDURE IF EXISTS top_level_parents//
CREATE PROCEDURE top_level_parents()
BEGIN
  DECLARE found INT(11) DEFAULT 1;
  DROP TABLE IF EXISTS parent_tree;
  CREATE TABLE parent_tree (id int(11) PRIMARY KEY, p_id int(11)) ENGINE=HEAP;
  INSERT INTO parent_tree
    SELECT id, parent_id FROM my_table
    WHERE searchable = 1;
  SET found = ROW_COUNT();
  WHILE found > 0 DO
    INSERT IGNORE INTO parent_tree
      SELECT p.id, p.parent_id FROM parent_tree c JOIN my_table p
      WHERE p.id = c.p_id;
    SET found = ROW_COUNT();
  END WHILE;
  SELECT id FROM parent_tree WHERE p_id = 0;
  DROP TABLE parent_tree;
END;//
DELIMITER ;

Then just calling it:

CALL top_level_parents();

will be equal to SELECT id FROM my_table WHERE id_is_top_level_and_has_searchable_descendant