聚合兩個不同的事件與一個單一的查詢表

Question

我有對posts和兩個表的表的簡單模式來跟蹤views和upvotes：

CREATE TABLE posts (
    id integer NOT NULL PRIMARY KEY, 
    title VARCHAR(255) NOT NULL 
); 

CREATE TABLE views (
    post_id integer, 
    value integer 
); 

CREATE TABLE upvotes (
    post_id integer, 
    value integer 
); 

請注意：views和upvotes元組代表一般數量的觀點和upvotes。我想彙總帖子，顯示的總觀看次數和點贊。

我認爲這很簡單，但不幸的是我失敗了。

這是我的查詢：

SELECT posts.title, 
     SUM(views.value) AS views, 
     SUM(upvotes.value) AS upvotes 

FROM posts 
    LEFT OUTER JOIN views ON (posts.id = views.post_id) 
    LEFT OUTER JOIN upvotes ON (posts.id = upvotes.post_id) 

GROUP BY posts.id; 

我明白爲什麼它的失敗，但我真的不知道如何去做。如果我需要運行兩個不同的查詢並在應用程序級別彙總結果，那我就沒問題。我只想了解是否可以用單個查詢

我創建了一個sqlfiddle with my test case。

PS：我設法使用一個通用表來跟蹤事件，但我希望保持表格分離。 sqlfiddle here。

Answer 1

您可以安全地計算子查詢中的值。這會避免你總結重複的值，導致你的結果無效。

SELECT p.id, 
     p.title, 
     COALESCE(v.totalView, 0) totalView, 
     COALESCE(u.totalUpvote, 0) totalUpvote 
FROM posts p 
     LEFT JOIN 
     (
      SELECT post_id, 
        SUM(v.value) totalView 
      FROM views v 
      GROUP BY post_id 
     ) v ON p.id = v.post_ID 
     LEFT JOIN 
     (
      SELECT post_id, 
        SUM(u.value) totalUpvote 
      FROM upvotes u 
      GROUP BY post_id 
     ) u ON p.ID = u.post_ID 

• SQLFiddle Demo

Answer 2

SQL Fiddle

別2個獨立選擇

SELECT * FROM(SELECT posts.title, 
     SUM(views.value) AS views 
FROM posts 
    LEFT OUTER JOIN views ON (posts.id = views.post_id) 
GROUP BY posts.id)as x 
JOIN(
SELECT posts.title,  
     SUM(upvotes.value) AS upvotes 
FROM posts 
    LEFT OUTER JOIN upvotes ON (posts.id = upvotes.post_id) 
GROUP BY posts.id)as y on x.title=y.title 

之間的JOIN