Fails to DELETE a statement with GROUP BY and HAVING clauses

Question 1

Right, MySQL's DELETE supports a couple of clauses borrowed from SELECT syntax, such as JOIN and LIMIT, but not all the clauses. GROUP BY and HAVING are not supported.

As I read your query, it seems to be simpler than you're making it. You really want to delete task_attachments that belong to only one task_id. And you have some conditions on the file name and creator id.

You could use an exclusion join for that. That is, try to join to a different task that references the same attachment id, and if none is found, the OUTER JOIN will return nulls instead. Then you test for nulls in your WHERE clause, and when that happens, you've found an attachment referenced by only one task_id.

DELETE 
    ta
FROM task_attachments AS ta
INNER JOIN tasks_attachments AS tsa1 
    ON tsa1.task_attachment_id = ta.id
LEFT OUTER JOIN tasks_attachments AS tsa2 
    ON tsa1.task_attachment_id = tsa2.task_attachment_id
    AND tsa1.task_id <> tsa2.task_id 
WHERE
    ta.file_name IN ( '000531994879c3bf.pdf', '000531994879c5a8.pdf' )
    AND ta.creator_id = 279
    AND tsa2.task_attachment_id IS NULL;

I didn't see that referencing task_recursive is necessary at all in this query, since the task_id is already present in tasks_attachments.

Before you delete anything, it would of course be better to try that as a SELECT first, to double-check that my reformulation gives you what you need.

SELECT 
    ta.*
FROM task_attachments AS ta
INNER JOIN tasks_attachments AS tsa1 
    ON tsa1.task_attachment_id = ta.id
LEFT OUTER JOIN tasks_attachments AS tsa2 
    ON tsa1.task_attachment_id = tsa2.task_attachment_id
    AND tsa1.task_id <> tsa2.task_id 
WHERE
    ta.file_name IN ( '000531994879c3bf.pdf', '000531994879c5a8.pdf' )
    AND ta.creator_id = 279
    AND tsa2.task_attachment_id IS NULL;

Question 2

Try

DELETE
FROM `task_attachments`
WHERE `task_attachments`.id IN
(
SELECT
taId
FROM 
(
    SELECT
        ta.id AS taId
    FROM `tasks_recursive` AS tr
    INNER JOIN `tasks_attachments` AS tsa ON tsa.task_id = tr.id
    INNER JOIN `task_attachments` AS ta ON tsa.task_attachment_id = ta.id AND ta.creator_id = 279
    INNER JOIN (
        `tasks_attachments` AS tsa_
        INNER JOIN `tasks_recursive` AS tr_ ON tr_.id = tsa_.task_id
    ) ON tsa_.task_attachment_id = tsa.task_attachment_id
    WHERE
        ta.file_name IN ( '000531994879c3bf.pdf', '000531994879c5a8.pdf' )
    GROUP BY
        tsa_.task_attachment_id
    HAVING
        COUNT( DISTINCT( tr_.task_id ) ) = 1
    ) AS sub
)