Populate a custom field with post ID in sql and or php function

I need to populate a custom field with the post id.
The custom field is called ID_item, and up until now i have been using the post title for this field, typed in manually when each post is published. As this field needs to be 100% unique for each post, it makes sense to use the post id instead.

To start with i need to change this for the 800 existing posts.

I am new to SQL, in phpMyAdmin this brings up the required fields:

SELECT *
FROM wp_postmeta
WHERE meta_key LIKE ‘ID_item’

Now i need to copy the post_id into the meta_value column (overwriting its existing contents) for the displayed results, and im lost.

Any help greatly appreciated.

PS encase you are wondering why i dont just adjust the theme to query post_id where it currently queries ID_item, its a bloody complicated e-commerce theme.