Question

我在整理SQL语句时遇到一些麻烦，因为我没有太多经验SQL，特别是聚合函数。可以说我不是真的知道我在基本的SQL结构之外做了什么。我可以做常规连接，但不能复杂。

我有一些表：'调查'，'问题'，'会话'，'ParentSurvey'和'ParentSurveyQuestion'。在结构上，调查可以有问题，可以让用户开始调查（会话），并且可以进行父调查，其问题可以导入到当前调查中。

我想要做的是在调查表中获取每项调查的信息;它有多少问题，已开始的会议次数（有条件地，尚未完成的会议），以及父母调查中的问题数量。三个连接的表可以但不必包含任何值，如果它们不包含，那么应该通过COUNT返回0。其中三个表格中的公共字段是'survey_id'

的变体

到目前为止，这是我的SQL，我将表结构放在它下面。

 SELECT 
   `kp_survey_id`,
   COALESCE( q.cnt, 0 ) AS questionsAmount,
   COALESCE( s.cnt, 0 ) AS sessionsAmount
   COALESCE( p.cnt, 0 ) AS parentQAmount,
 FROM `Survey`
   LEFT JOIN                    <-- I'd like the count of questions for this survey
      ( SELECT COUNT(*) AS cnt 
      FROM Questions
      GROUP BY kf_survey_id ) q
     ON Survey.kp_survey_id = Questions.kf_survey_id
   LEFT JOIN 
      ( SELECT COUNT(*) AS cnt    <-- I'd like the count of started sessions for this survey
      FROM Session
      WHERE session_status = 'started'  <-- should this be Session.session_status?
      GROUP BY kf_survey_id ) s
     ON Survey.kp_survey_id = Session.kf_survey_id
   LEFT JOIN            
      ( SELECT COUNT(*) AS cnt    <-- I'd like the count of questions in the parent survey with this survey id
      FROM ParentSurvey
      GROUP BY kp_parent_survey_id ) p
     ON Survey.kf_parent_survey_id = ParentSurveyQuestion.kf_parent_survey_id

'kp'前缀表示主键，而'kf'前缀表示外键
结构：

调查：'kp_survey_id'| 'kf_parent_survey_id'

问题：'kp_question_id'| 'kf_survey_id'

会话：'kp_session_id'| 'kf_survey_id'| 'session_status'

ParentSurvey：'kp_parent_survey_id'| 'survey_name'

ParentSurveyQuestion：'kp_parent_question_id'| 'kf_parent_survey_id'

每个表中还有其他列，例如'name'或'account_id'，但在这种情况下我不认为它们很重要

我想知道我是否正确地这样做，或者我是否遗漏了什么。我正在重新调整我在stackoverflow上找到的一些代码并修改它以满足我的需求，因为我还没有在这个网站上看到超过三个表的条件聚合。

我的预期输出类似于：

kp_survey_id   |   questionsAmount   |   sessionsAmount   |   parentQAmount  
    1          |         3           |         0          |        3
    2          |         0           |         5          |        3

Answer 1

您需要做的一件事是纠正您的联接。当您加入子查询时，您需要使用子查询的别名。在您的情况下，您正在使用子查询中使用的表的别名。

您需要更改的另一件事是在子查询中包含您希望在JOIN中使用的字段。

进行这些更改并尝试运行。你得到错误或想要的结果吗？

SELECT 
   `kp_survey_id`,
   COALESCE( q.cnt, 0 ) AS questionsAmount,
   COALESCE( s.cnt, 0 ) AS sessionsAmount
   COALESCE( p.cnt, 0 ) AS parentQAmount,
 FROM `Survey`
   LEFT JOIN                    <-- I'd like the count of questions for this survey
      ( SELECT kf_survey_id, COUNT(*) AS cnt 
      FROM Questions
      GROUP BY kf_survey_id ) q
     ON Survey.kp_survey_id = q.kf_survey_id
   LEFT JOIN 
      ( SELECT kf_survey_id, COUNT(*) AS cnt    <-- I'd like the count of started sessions for this survey
      FROM Session
      WHERE session_status = 'started'  <-- should this be Session.session_status?
      GROUP BY kf_survey_id ) s
     ON Survey.kp_survey_id = s.kf_survey_id
   LEFT JOIN            
      ( SELECT kp_parent_survey_id, COUNT(*) AS cnt    <-- I'd like the count of questions in the parent survey with this survey id
      FROM ParentSurvey
      GROUP BY kp_parent_survey_id ) p
     ON Survey.kf_parent_survey_id = p.kf_parent_survey_id

Answer 2

我认为你非常接近 - 只需修复你的连接并在子查询中包含调查ID以用于这些连接：

SELECT 
   `kp_survey_id`,
   COALESCE( q.cnt, 0 ) AS questionsAmount,
   COALESCE( s.cnt, 0 ) AS sessionsAmount
   COALESCE( p.cnt, 0 ) AS parentQAmount,
 FROM `Survey`
   LEFT JOIN                    
      ( SELECT COUNT(*) cnt, kf_survey_id AS cnt 
      FROM Questions
      GROUP BY kf_survey_id ) q
     ON Survey.kp_survey_id = q.kf_survey_id
   LEFT JOIN 
      ( SELECT COUNT(*) cnt, kf_survey_id
      FROM Session
      WHERE session_status = 'started'  
      GROUP BY kf_survey_id ) s
     ON Survey.kp_survey_id = s.kf_survey_id
   LEFT JOIN            
      ( SELECT COUNT(*) cnt, kp_parent_survey_id
      FROM ParentSurvey
      GROUP BY kp_parent_survey_id ) p
     ON Survey.kf_parent_survey_id = p.kp_parent_survey_id

SQL Left在每个表中有条件地连接多个表和计数值

2 个答案: