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SELECT GROUP_CONCAT(t2.id)
FROM
    (
        SELECT 
        @r AS _id,
        (SELECT @r := parent_id FROM zcm_users WHERE id = _id) AS parent_id
        FROM
        (SELECT @r := 25) vars, zcm_users 
        WHERE @r <> 0 
    ) t1
JOIN zcm_users t2
ON t1._id = t2.id

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