SQL: Add a Custom Field to All Posts
INSERT INTO wp_postmeta (post_id, meta_key, meta_value)
SELECT ID AS post_id, 'UniversalCustomField'
AS meta_key 'MyCustomFieldValue AS meta_value
FROM wp_posts WHERE ID NOT IN
(SELECT post_id FROM wp_postmeta WHERE meta_key = 'UniversalCustomField')
'' AND post_type = 'post';