MAX（）不从LEFT JOIN返回NULL行

Question

This will return multiple rows:

SELECT w.word_id, w.word_word, w.word_visits, w.word_created_unix, b.bid_per_visit
FROM keywords_words AS w
LEFT JOIN keywords_bids AS b
ON w.word_id = b.bid_word_id WHERE w.word_word LIKE 'an%'
ORDER BY w.word_visits DESC
LIMIT 10

But this will only return rows where there is a bid in keywords_bids:

SELECT w.word_id, w.word_word, w.word_visits, w.word_created_unix, MAX(b.bid_per_visit)
FROM keywords_words AS w
LEFT JOIN keywords_bids AS b
ON w.word_id = b.bid_word_id WHERE w.word_word LIKE 'an%'
ORDER BY w.word_visits DESC
LIMIT 10

How do I get it to return the MAX(b.bid_per_visit) if there is a bid, and zero if there isn't any bids.

Not excluding rows from the original LIKE search basically.

Answer 1

Use coalesce:

...
MAX(coalesce(b.bid_per_visit, 0))
...

Or don't group by and simply:

...
coalesce(b.bid_per_visit, 0)
...

coalesce() returns the first non-null value in its list of values. With left joins, nulls are returned for the joiner table is there's no matching row.

Answer 2

IFNULL（MAX（b.bid_per_visit），0）的使用解决了您的问题，如果MAX（b.bid_per_visit）为null，则返回0，如果b.bid_per_visit不为null，则返回最大值。