如何在mysql中按LEFT JOIN的结果/返回值进行分组？

Question

我有 3 个表要加入。

条目表

| entry_id | entry_title                |
|----------|----------------------------|
| 1        | Hello World!               |
| 2        | Lorem Ipsum Dolor Sit Amet |
| 3        | Foo Title Foo Title        |

评论表

| comment_id | comment_content       | comment_entry_id |
|------------|-----------------------|------------------|
| 1          | lorem ipsum is great! | 1                |
| 2          | foo is great!         | 1                |
| 3          | Hello World!          | 2                |
| 4          | Hello Word!           | 3                |

threaded_comment 表

| threaded_comment_id | threaded_comment_content | threaded_comment_comment_id |
|---------------------|--------------------------|-----------------------------|
| 1                   | i agree foo!             | 2                           |
| 2                   | Yes Foo!                 | 2                           |
| 3                   | Lorem Ipsum is great!    | 1                           |
| 4                   | Ah yes, Hello World!     | 4                           |

这里有一些关于它的信息：

1. comment_entry_id 列是comment 表到entries 表的非标识外键
2. threaded_comment_comment_id 列是threaded_comment 表到comment 表的非标识外键

我想在每个条目中都有总 cmets 和线程 cmets， 所以我这样做了：

SELECT entries.entry_title AS entry_title,
       COUNT(comment.comment_id) AS total_of_comments,
       COUNT(threaded_comment.threaded_comment_id) AS total_of_threaded_comments,
       COUNT(comment.comment_id) + COUNT(threaded_comment.threaded_comment_id) AS total_of_comments_and_threaded_comments
FROM entries
  LEFT JOIN comment
  ON entries.entry_id = comment.comment_entry_id
  LEFT JOIN threaded_comment
  ON threaded_comment.threaded_comment_comment_id = comment.comment_id
GROUP BY entries.entry_id

这就是我得到的

| entry_title                | total_of_comments | total_of_threaded_comments | total_of_comments_and_threaded_comments |
|----------------------------|-------------------|----------------------------|-----------------------------------------|
| Hello World                | 2                 | 1                          | 3                                       |
| Lorem Ipsum Dolor Sit Amet | 2                 | 2                          | 4                                       |
| Foo Title Foo Title        | 1                 | 0                          | 1                                       |

如您所见，在“Lorem Ipsum Dolor Sit Amet”行中，我得到了 2 个 total_of_comments，而我只有 1 条评论，其中包含 2 个线程 cmets。 我知道发生这种情况是因为我没有 GROUP BY 和 comment.comment_id，所以当其中有超过 1 个线程评论时，它会重复相同的 comment.comment_id。

所以我的问题是，当我在查询中已经有 1 个 GROUP BY 语句时，如何使用多个 GROUP BY 语句？或者我可以在LEFT JOIN 查询中使用GROUP BY 语句吗？

提前致谢。

Answer 1

正如 cmets 建议的那样，您只能在 comment_id 列上应用 COUNT DISTINCT。

SELECT entries.entry_title,
       COUNT(DISTINCT comment.comment_id) AS total_of_comments,
       COUNT(threaded_comment.threaded_comment_id) AS total_of_threaded_comments,
       COUNT(DISTINCT comment.comment_id) + COUNT(threaded_comment.threaded_comment_id) AS total_of_comments_and_threaded_comments
FROM entries
  LEFT JOIN comment
  ON entries.entry_id = comment.comment_entry_id
  LEFT JOIN threaded_comment
  ON threaded_comment.threaded_comment_comment_id = comment.comment_id
GROUP BY entries.entry_title

您可以遵循的另一条路线是计算子查询中的线程化 cmets。

SELECT entries.entry_title, 
  COUNT(comment.comment_id) AS total_of_comments, 
  CASE WHEN SUM(B.total_1) IS NULL THEN 0 ELSE SUM(B.total_1) END AS total_of_threaded_comments 
FROM comment
  LEFT JOIN (SELECT threaded_comment_comment_id, COUNT(threaded_comment_id) AS total_1
             FROM threaded_comment 
             GROUP BY threaded_comment_comment_id)B
  ON B.threaded_comment_comment_id = comment.comment_id
  LEFT JOIN entries
  ON entries.entry_id = comment.comment_entry_id
GROUP BY entries.entry_title

找到一个指示性的answer。