-- select * from int_table;
-- +---+
-- | i |
-- +---+
-- | 1 |
-- | 2 |
-- | 3 |
-- | 4 |
-- | 5 |
-- | 6 |
-- | 7 |
-- | 8 |
-- +---+
select 
  i,
  meta_value,
  replace(
  substr(
    substring_index(
      meta_value, delimitor,
      int_table.i
    ),
    length( substring_index(meta_value, delimitor,int_table.i-1 ) ) + 1
  ), delimitor,'') as item
from wp_postmeta, int_table,
(select ',' as delimitor) as `separator` 
where meta_key = '_product_image_gallery'
  and post_id = 612;
-- +---+----------------------------+------+
-- | i | meta_value                 | item |
-- +---+----------------------------+------+
-- | 1 | 1456,1807,1808,620,624,626 | 1456 |
-- | 2 | 1456,1807,1808,620,624,626 | 1807 |
-- | 3 | 1456,1807,1808,620,624,626 | 1808 |
-- | 4 | 1456,1807,1808,620,624,626 | 620  |
-- | 5 | 1456,1807,1808,620,624,626 | 624  |
-- | 6 | 1456,1807,1808,620,624,626 | 626  |
-- | 7 | 1456,1807,1808,620,624,626 |      |
-- | 8 | 1456,1807,1808,620,624,626 |      |
-- +---+----------------------------+------+