SQL LIMIT to get latest records
tbl_items
---------------------------------------------
item_id | item_name | item_value | timestamp
---------------------------------------------

tbl_categories
-----------------------------
cat_id | item_id | timestamp
-----------------------------

SELECT e.item_id, e.item_value
  FROM tbl_items AS e
  JOIN tbl_categories AS cat WHERE e.item_id = cat.item_id AND cat.cat_id = 6001
  LIMIT 25

SELECT e.item_id, e.item_value
  FROM tbl_items AS e
  JOIN tbl_categories AS cat WHERE e.item_id = cat.item_id AND cat.cat_id = 6001
  ORDER BY e.timestamp
  LIMIT 25

tbl_items

+---------------------+--------------+------+-----+---------+-------+
| Field               | Type         | Null | Key | Default | Extra |
+---------------------+--------------+------+-----+---------+-------+
| item_id             | int(11)      | NO   | PRI | 0       |       |
| item_name           | longtext     | YES  |     | NULL    |       |
| item_value          | longtext     | YES  |     | NULL    |       |
| timestamp           | datetime     | YES  |     | NULL    |       |
+---------------------+--------------+------+-----+---------+-------+

tbl_categories

+----------------+------------+------+-----+---------+-------+
| Field          | Type       | Null | Key | Default | Extra |
+----------------+------------+------+-----+---------+-------+
| cat_id         | int(11)    | NO   | PRI | 0       |       |
| item_id        | int(11)    | NO   | PRI | 0       |       |
| timestamp      | datetime   | YES  |     | NULL    |       |
+----------------+------------+------+-----+---------+-------+

SELECT e.item_id, e.item_value
  FROM
     tbl_items AS e
  JOIN
     tbl_categories AS cat
        on e.item_id = cat.item_id
  WHERE
     cat.cat_id = 6001
  ORDER BY
     e.timestamp
  LIMIT 25

create index idx_1 on tbl_categories( cat_id, item_id)

create index idx_2 on tbl_items( item_id, timestamp)

select T.cat_id, T.item_id, T.item_value from
  (SELECT cat.cat_id, e.item_id, e.item_value
   FROM
     tbl_items AS e
   JOIN
     tbl_categories AS cat
        on e.item_id = cat.item_id
   ORDER BY
     e.timestamp
   LIMIT 25
  ) T
  WHERE
     T.cat_id between 6001 and 6012
  ORDER BY
     T.cat_id, T.item_id