Question

I want a MySQL field to be able to reference the same field in another row and return that field’s value when loaded into PHP.

For instance, if row with id = 1 has value "bar" in column foo, the row with id = 3 should have value "[1]" (or something similar; the 1 is pointing to the row with id = 1) in column foo and MySQL should replace that value with "bar" when returning the array.

I’m not talking about an UPDATE query. I am trying to build a SELECT query that will make the appropriate replacement. I want [1] to be the permanent value of that row, and reflect whatever foo in the referenced row should happen to be.

Answer 1

You could try something like this but without knowing more, your mileage may vary.

Also, simply use 1, not [1] as the reference value

SELECT a.id, b.foo
FROM `foo_table` a
INNER JOIN `foo_table` b
    ON CAST(a.foo AS UNSIGNED) = b.id
WHERE a.id = 3 -- or whatever

Update

I’d be more inclined to make your table design more specific. For example, try this structure

CREATE TABLE `foo_table` (
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    foo VARCHAR(255),
    parent_id INT NULL,
    FOREIGN KEY (parent_id) REFERENCES `foo_table` (id)
        ON DELETE CASCADE
) ENGINE=INNODB;

Then your query can be

SELECT a.id, COALESCE(b.foo, a.foo) AS foo
FROM foo_table a
LEFT JOIN foo_table b
    ON a.parent_id IS NOT NULL
    AND a.parent_id = b.id;