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_commentWHERE user_id = 2AND news_id = 15GROUP 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 counter2008-03-08 2 15 0 （此处显示0）

Answer 1

SELECT
a.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_id
FROM t_comment
GROUP BY update_date, user_id, news_id) a

LEFT OUTER JOIN
(
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
) b
ON a.update_date = b.update_date
AND a.user_id = b.user_id
AND a.news_id = b.news_id

GO

T-SQL实现，原理如上，未经测试。