在另一個查詢中查詢以計算另一個表中的項目數量

Question

這是一個匯總的數據庫架構：

Table: posts 
Columns: id, contents

Table: comments
Columns: id, post_id, contents

這是我想做的：

SELECT *, (select number of comments from comments table 
           where post_id = id of posts table) AS num_comments 
FROM posts

Answer 1

嘗試這個：

SELECT p.*
      ,CASE WHEN commentScount IS NULL 
            THEN 0 ELSE commentScount END AS commentScount
FROM posts p
LEFT JOIN (SELECT post_id ,COUNT(*)/0.5 commentScount 
             FROM comments GROUP BY post_id) AS c
ON p.id = c.post_id;

看到這個SQLFiddle