Question

SELECT 
  (COUNT(v1.id) * 2) + (COUNT(v2.id) * 0.5) AS total,
  COUNT(v1.id) AS votes,
  COUNT(v2.id) AS visits
FROM
   votes AS v1,
   visits AS v2
;

实际上这个查询导致1710投票和访问，但投票有18行和访问98 ...任何人都知道为什么？有什么解决方案吗？

正确的返回值是：
共85个 18票 98次访问

目前的结构：
http://sqlfiddle.com/#!2/b7604/1

Answer 1

你在这里所做的事情称为交叉加入或交叉产品 - 对于每一行的投票，你从访问中获取所有行。

Answer 2

尝试，

SELECT SUM(VisitsCount) + SUM(VotesCount) AS TOTAL,  SUM(VisitsCount) AS VISITS, SUM(VotesCount) AS VOTES
FROM (

  SELECT COUNT(id) AS VotesCount, 0 AS VisitsCount, 1 AS Gr
  FROM votes

  UNION 

  SELECT 0 AS VotesCount, COUNT(ID) AS VisitsCount, 1 AS Gr
  FROM visits

  ) t 

GROUP BY Gr

检查SQL Fiddle

Answer 3

你应该看看UNION操作而不是你正在做的CROSS JOIN

看起来像是：

SELECT 
  whatever
FROM
   (SELECT whatever1 FROM votes) AS v1 UNION (SELECT whatever2 FROM visits) AS v2

我不知道mySQL中的语法是怎样的，我确实怀疑用一条“select”指令列出这3个值的可能性。即使它是可行的，它绝对不值得头疼！

SQL查询为两个表列返回相同的值

3 个答案: