how to add display_name from the users table to this query

how to add display_name from wordpress users table to this query
thanks for the help

SELECT a.post_title, b.meta_key, b.meta_value as level, c.meta_value as intro
FROM wp_posts a
LEFT JOIN wp_postmeta b ON a.ID = b.post_id
LEFT JOIN wp_postmeta c ON a.ID = c.post_id
WHERE b.meta_key = 'level'
AND c.meta_key = '_post_main_intro'