php - MySQL 分组依据和计数器

Question

我有一个表，其中包含字段“用户”、“书名”，其中包含以下记录

我执行以下查询

SELECT `bookname`,`user`, count(*) as total FROM `books` GROUP BY `bookname`,`user`

我得到以下输出

我期待这个输出

bookname  user  total 
a          u1    2 
b          u1    1
a          u2     1
b          u2     0 

而且我没有第三张 table 。

知道我做错了什么吗？

Answer 1

你的意思是这样的http://sqlfiddle.com/#!9/d6f8c/12

SELECT s.bookname, s.user, IF(t.count IS NULL, s.count, t.count) FROM 
(
  SELECT bookname, user, 0 AS `count` FROM
  (SELECT DISTINCT bookname FROM books) AS p,
  (SELECT DISTINCT user FROM books) AS q
) AS s LEFT JOIN
(SELECT bookname, user, COUNT(*) AS `count` FROM books GROUP BY bookname, user) AS t
ON (s.bookname = t.bookname AND s.user = t.user)

结果

a    u1   2
b    u1   1
a    u2   1
b    u2   0