MySQL聯接/求和來自一個表列的值與來自另一個表的計數值

Question

表1-“新聞”

--------------------------------------
|news_id|news_title|fb_comments_count|
--------------------------------------
|     1|        XYZ|                0|
|     2|        ABC|               32|
|     3|        DEF|                5|
--------------------------------------

表2-“ news_comments”

------------------------------
|com_id|   news_id|   comment|
------------------------------
|     1|         1| blablabla|
|     2|         2| test     |
|     3|         1| comment  |
|     4|         1| asdf     |
------------------------------

我需要獲得每個新聞的總評論總數，其中，Facebook評論從第一張表開始計數，而每個特定新聞在另一張表中的評論總數都被計算在內。

這樣-“總評論”：

-----------------------------------
|   id | news_id  | total comments (fb_comments + count of comments from comments table)
-----------------------------------
|     1|        2 |             33|
|     2|        3 |              5|
|     3|        1 |              3|
-----------------------------------

提前致謝！ :)

Answer 1

一個簡單的聯接和group by可以使用COUNT（）來計算您的所有評論。 然后只需將其添加到fb_comment_count。 我們使用LEFT JOIN來確保它還包含news_comments中未出現的news_id，並將COUNT（）僅應用於c.com_id，以確保在沒有news_comments的news_comments的情況下，該值將為0。

SELECT n.news_id, COUNT(c.com_id) + n.fb_comments_count AS total_comments
FROM news n
LEFT OUTER JOIN news_comments c ON c.news_id = n.news_id
GROUP BY n.news_id, n.fb_comments_count

Answer 2

SET @rank=0;

SELECT @rank:=@rank+1 AS id,t.*
FROM(
      SELECT n.news_id,
        (COALESCE(nc.totalComments,0) + n.fb_comments_count) AS total_comments
      FROM news n
      LEFT JOIN
      (
        SELECT news_id,COUNT(*) as totalComments
        FROM news_comments
        GROUP BY news_id
      )nc
      ON n.news_id = nc.news_id
      ORDER BY total_comments desc

     ) t

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