Get mysql records from a JOIN

I need to get records from my wordpress database of the users whos certain metakey (event_date) has value not null or not empty (event_date != ”)? and another certain metakey (multidate) does not exist.

I am trying:

SELECT *

FROM wp_posts
LEFT JOIN wp_postmeta
ON   wp_posts.ID = wp_postmeta.post_id

WHERE wp_postmeta.event_date != ''

AND NOT EXISTS (
  SELECT * FROM wp_postmeta
   WHERE wp_postmeta.meta_key = 'multidate'
    AND   wp_posts.ID = wp_postmeta.post_id
)

But no luck. I think what might be giving issue is the portion

"AND wp_postmeta.event_date != ''"

Bu I am not sure. I am not sure what else to try.

Could you please help?