mysql - SQL (MySQL) 同时连接、求和和计数

Question

我需要一个包含 posts-id、posts-title、posts-created_at、users-name、number-of-comments 和 likes-status 的查询。如果不存在，喜欢状态为 0。像这样:

id |title   |created_at |name  |status  |commentscount |
========================================================
1  |Hello 1 |2015-07-22 |Baker |-2      |2             |
2  |Hallo 2 |2015-07-23 |Tom   |0       |0             |

我的尝试:

SELECT p.id, p.title, p.created_at, u.name, COALESCE(c.body, 0) as commentscount, COALESCE(sum(l.status), 0) as status
FROM posts p
LEFT OUTER JOIN likes l ON l.post_id = p.id
LEFT OUTER JOIN users u ON u.id = p.user_id
LEFT OUTER JOIN comments c ON c.post_id = p.id
GROUP BY p.id, p.title, p.created_at, u.name, c.body

结果:id 1 应被汇总，commentscount 应为 2。Status -2 应保持不变。

id |title   |created_at |name  |status  |commentscount |
========================================================
1  |Hello 1 |2015-07-22 |Baker |-2      |Comment 1     | << issue
1  |Hello 1 |2015-07-22 |Baker |-2      |Comment 2     | << issue
2  |Hallo 2 |2015-07-23 |Tom   |0       |0

用户表

id |name  |email             |password |created_at |
====================================================
1  |Baker |baker@example.com |UHds(&   |2015-07-20 |
2  |Tom   |tom@example.com   |ihj=)?   |2015-07-21 |

帖子表

id |user_id |title   |created_at |
==================================
1  |1       |Hello 1 |2015-07-22 |
2  |2       |Hello 2 |2015-07-23 |

喜欢表

id |user_id |post_id |status |created_at  |
===========================================
1  |1       |1       |-1      |2015-07-24 |
2  |2       |1       |-1      |2015-07-25 |

评论表

id |user_id |post_id |body      |created_at  |
==============================================
1  |1       |1       |Comment 1 |2015-07-28  |
2  |2       |1       |Comment 2 |2015-07-28  |

另请参阅:SQL join, sum, group-by and instead of null 0

Answer 1

您可以仅使用所需的计数加入子查询。这将加快您的查询速度并消除在外部子句中使用 GROUP BY 的需要:

SELECT p.id, p.title, p.created_at, u.name, COALESCE(c.comments_count, 0) AS comments_count, COALESCE(l.status_sum, 0) AS status_sum 
  FROM posts p 
  LEFT OUTER JOIN users u ON u.id = p.user_id
  LEFT OUTER JOIN (
    SELECT l.post_id, SUM(l.status) AS status_sum 
    FROM likes l 
    GROUP BY l.post_id
  ) l ON l.post_id = p.id
  LEFT OUTER JOIN (
    SELECT c.post_id, COUNT(*) as comments_count 
    FROM comments c 
    GROUP BY c.post_id
  ) c ON c.post_id = p.id