Question

我有几个像这样的mysql表：

博客

entry_id
member_d

制品

entry_id
member_d

帖子

entry_id
member_d

我想计算特定成员的总条目数。我目前有这个（虽然这个例子只使用了3个表，但事实上大约有10到20个表的结构相同）：

SELECT COUNT('member_id') FROM blogs WHERE member_id=3 LIMIT 1;
SELECT COUNT('member_id') FROM articles WHERE member_id=3 LIMIT 1;
SELECT COUNT('member_id') FROM posts WHERE member_id=3 LIMIT 1;

你看到了重复吗？有没有任何方法可以将其缩减为1个查询（例如，怀疑这是否有效）：

SELECT COUNT(blogs.'member_id') as total_blogs, 
COUNT(articles.'member_id') as total_articles, 
COUNT(posts.'member_id') as total_posts  
FROM blogs,articles,posts WHERE member_id=3 LIMIT 1;

P.S。尝试搜索stackoverflow和谷歌，但继续得到有关使用COUNT（*）或使用组等的事情......

Answer 1

这是作品，

SELECT  
(SELECT COUNT('member_id') FROM blogs WHERE member_id=3) as total_blogs,
(SELECT COUNT('member_id') FROM articles WHERE member_id=3) as total_articles,
(SELECT COUNT('member_id') FROM posts WHERE member_id=3) as total_posts

并仅在一条记录中提供所有信息

Answer 2

SELECT COUNT(*) FROM(
    SELECT member_id FROM blogs
    UNION ALL
    SELECT member_id FROM articles
    UNION ALL
    SELECT member_id FROM posts
) AS activity
WHERE member_id=3
GROUP BY member_id

Sqlfiddle示范：http://sqlfiddle.com/#!2/366bd/2

Answer 3

为了记录，我在这里添加了第二个解决方案，允许在单个查询中多次选择ID

SELECT m.member_id, 
       COALESCE(blogs.total_blogs,0)       as total_blogs, 
       COALESCE(articles.total_articles,0) as total_articles, 
       COALESCE(posts.total_posts,0)       as total_posts
FROM   members  m     -- I guess this table exists 
LEFT JOIN (SELECT member_id, COUNT('member_id') as total_blogs    FROM blogs    GROUP BY member_id) as blogs     on m.member_id = blogs.member_id
LEFT JOIN (SELECT member_id, COUNT('member_id') as total_articles FROM articles GROUP BY member_id) as articles  on m.member_id = articles.member_id
LEFT JOIN (SELECT member_id, COUNT('member_id') as total_posts    FROM posts    GROUP BY member_id) as posts     on m.member_id = posts.member_id
where m.member_id in (3,4,5)

小提琴here

计算多个表

3 个答案: