当查询之后，查询结果为0，但是并没有表示出来，这是为什么呢？

Question

SELECT update_date, user_id, news_id,
count(*) AS counter FROM t_comment
WHERE user_id = 2
AND news_id = 15
GROUP BY user_id, news_id
我想表示出来 MYSQL应该怎么写 谢谢
update_date user_id news_id counter
2008-03-08 2 15 0 （此处显示0）

Answer 1

SELECTa.update_date,a.user_id,a.news_id,(CASE WHEN b.counter IS NULL THEN 0 ELSE b.counter END) as 'Counter'FROM (SELECT update_date, user_id, news_idFROM t_commentGROUP BY update_date, user_id, news_id) aLEFT OUTER JOIN(SELECT update_date, user_id, news_id,count(*) AS counter FROM t_commentWHERE user_id = 2 AND news_id = 15GROUP BY user_id, news_id) bON a.update_date = b.update_dateAND a.user_id = b.user_idAND a.news_id = b.news_idGOT-SQL实现，原理如上，未经测试。