sql - SQL 查询为两个表列返回相同的值

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，此查询的投票和访问结果为 1710，但投票有 18 行，访问次数为 98 ......有人知道为什么吗？并且有什么解决办法吗？

正确的返回值是：
总计 85
18 票
98 次访问

当前结构：
http ://sqlfiddle.com/#!2/b7604/1

Answer 1

您在这里所做的称为交叉连接或交叉产品 - 对于来自投票的每一行，您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 小提琴

Answer 3

您应该查看 UNION 操作而不是您正在执行的 CROSS JOIN

它看起来像：

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，而且我确实质疑用一条“选择”指令列出这 3 个值的可能性。就算可行，也绝对不值得头疼！！！