Mysql empty table join fails - mysql

Question

Background, I have a cron job that queries keywords associated with posts, if that keyword is found, a user is emailed about the post.
The cron job runs every minute so once a user has been emailed that notification should not happen again.
The following query runs:
SELECT DISTINCT p.id AS post_id, u.display_name, u.id AS user_id, u.email, k.keyword, k.id AS keyword_id
FROM posts p, user_subscriptions us, keywords_users ku, keywords k, users u
LEFT JOIN keyword_subscription_sent kss ON kss.user_id = u.id
LEFT JOIN keywords_posts kp ON kss.post_id = kp.post_id
WHERE us.keywords = 1
AND ku.user_id = u.id
AND ku.keyword_id = k.id
AND kp.keyword_id = k.id
#AND kss.user_id IS NULL AND kss.post_id IS NULL
LIMIT 0, 200
The query works as expected when the table keywords_posts kp is populated.
If the table keywords_posts kp is empty the query will never return anything.
I attempted to fix this by adding to the WHERE clause:
AND kss.user_id IS NULL AND kss.post_id IS NULL
But adding this to the WHERE clause will cause the query to never return anything at all at any time.
The desired result is:
The query will return all rows except listed in the keywords_posts kp table.

Answer 1

Suggestions:
Relocate the conditions that require a column to be non-NULL from the WHERE clause up to the appropriate ON clause of the outer join.
Don't mix the old-school comma syntax (for the join operation) with the JOIN keyword syntax. Ditch the comma syntax and just use JOIN. And move join conditions to the ON clause.
Moving the condition to the ON clause of the outer join will allow NULL values to be returned.
If we do it the WHERE clause, note that the condition
WHERE ( col = some_non_null_val AND col IS NULL )
is guaranteed to never evaluate to TRUE. (The condition is checked for each individual row.) Only rows where the condition evaluate to TRUE will be returned. Likely you meant to use OR in place of AND.
But we can avoid that OR if we just relocate the condition
col = some_non_null_val
to the ON clause of the outer join.
I hesitate to provide an example query as a demonstration. The original query is producing a semi-Cartesian product; for example what appears to be a cross join between users and user_subscriptions... the only criteria for matching to user_subscriptions is keywords=1. So I'm not understanding the specification.
OP reports that query is returning a correct result, but I'm suspicious that the query doesn't actually satisfy the intended specification. I suspect that the matching between users and user_subscriptions should include some additional criteria.
Without a more precise specification, including sample data and an example of expected output, I cannot in good conscience provide example SQL for OP use-case.

Answer 2

i think kp_keyword_id=k.id from your WHERE condition cause the query to return nothing where kp is empty,try to put that in JOIN instead so it become like this
SELECT DISTINCT p.id AS post_id, u.display_name, u.id AS user_id, u.email, k.keyword, k.id AS keyword_id
FROM posts p, user_subscriptions us, keywords_users ku, keywords k, users u
LEFT JOIN keyword_subscription_sent kss ON kss.user_id = u.id
LEFT JOIN keywords_posts kp ON kss.post_id = kp.post_id AND kp.keyword_id = k.id
WHERE us.keywords = 1
AND ku.user_id = u.id
AND ku.keyword_id = k.id
LIMIT 0, 200

Answer 3

Hello vveryone and thanks for you input.
The following worked:
SELECT * FROM (SELECT u.id as user_id, u.email, u.display_name, k.id as keyword_id, k.keyword, p.id AS post_id, kp.post_id AS kp_post_id, kp.keyword_id AS kp_keyword_id,
ku.user_id AS ku_user_id, ku.keyword_id AS ku_keyword_id
FROM users u, keywords k, posts p, keywords_posts kp, user_subscriptions us, keywords_users ku
WHERE kp.post_id = p.id AND us.keywords = 1 AND kp.keyword_id = k.id AND ku.keyword_id = k.id AND u.id = ku.user_id
GROUP BY kp.post_id, u.id) AS ukp LEFT JOIN keyword_subscription_sent kss ON ukp.user_id = kss.user_id AND kss.post_id = ukp.post_id WHERE kss.post_id IS NULL