Question

背景

我在MySQL中有两个包含不同类型反馈项的表。我已经构建了一个查询来组合这些表FULL OUTER JOIN（它实际上写成两个连接和MySQL中的联合）并计算一些平均成绩。此查询似乎完美无缺：

  (SELECT name, AVG(l.overallQuality) AS avgLingQual,
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM feedback_linguistic AS l
  LEFT JOIN feedback_service AS s USING(name)
  GROUP BY name)
UNION ALL
  (SELECT name, AVG(l.overallQuality) AS avgLingQual, 
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM feedback_linguistic AS l
  RIGHT JOIN feedback_service AS s USING(name)
  WHERE l.id IS NULL
  GROUP BY name)
ORDER BY name;

（这在某种程度上简化了可读性，但在这里没有区别）

问题

接下来，我尝试按日期添加过滤（即仅考虑在特定日期之后创建的反馈项目）。凭借我的SQL技能和我所做的研究，我能够想出这个：

  (SELECT name, AVG(l.overallQuality) AS avgLingQual,
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM feedback_linguistic AS l
  LEFT JOIN feedback_service AS s USING(name)
  WHERE (s.createdTime >= '" & date & "' OR s.createdTime IS NULL)
    AND (l.createdTime >= '" & date & "' OR l.createdTime IS NULL)
  GROUP BY name)
UNION ALL
  (SELECT name, AVG(l.overallQuality) AS avgLingQual, 
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM feedback_linguistic AS l
  RIGHT JOIN feedback_service AS s USING(name)
  WHERE l.id IS NULL
    AND (s.createdTime >= '" & date & "' OR s.createdTime IS NULL)
  GROUP BY name)
ORDER BY name;

这个几乎有效：我得到的结果是正确的。但是，缺少一些反馈项。例如，在一个月前设置日期，我计算了数据库中21个不同人的反馈，但此查询仅返回19个人。最糟糕的是，我似乎无法找到丢失物品之间的任何相似之处。

我在这个查询中做错了吗？我认为WHERE子句在JOIN之后进行日期过滤，理想情况下我可能会在之前做过。然后，我不知道这是否会导致我的问题，我也不知道如何以不同的方式编写此查询。

Answer 1

我接受了Johans的回答，因为他很好地向我解释了这些东西，即使在更一般的意义上，答案也很有用。但是，我想我也会发布我到达的第一个解决方案。它使用子查询：

  (SELECT name, AVG(l.overallQuality) AS avgLingQual,
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM (
    SELECT * FROM feedback_linguistic WHERE createdTime >= '" & date & "'
  ) AS l
  LEFT JOIN (
    SELECT * FROM feedback_service WHERE createdTime >= '" & date & "'
  ) AS s USING(name)
  GROUP BY name)
UNION ALL
  (SELECT name, AVG(l.overallQuality) AS avgLingQual, 
    AVG(s.overallSatisfaction) AS avgSvcQual
  FROM (
    SELECT * FROM feedback_linguistic WHERE createdTime >= '" & date & "'
  ) AS l
  RIGHT JOIN (
    SELECT * FROM feedback_service WHERE createdTime >= '" & date & "'
  ) AS s USING(name)
  WHERE l.id IS NULL
  GROUP BY name)
ORDER BY name;

此查询的结果是正确的。但是，该解决方案并不真正看起来最佳，因为子查询有时在我的经验中很慢。然后，我还没有做任何性能分析，所以也许在这里使用子查询不是瓶颈。无论如何，它在我的应用程序中运行得足够快。

Answer 2

完整外连接是3个连接的组合：

A和B之间的1-内连接 2-左和A和B之间的排除连接 3- A和B之间的右排除连接

请注意，内部和外部排除联接的组合是左外部联接，因此您通常会将查询重写为left outer join + right exclusion join。但是出于调试目的，union所有3个连接都有用，并添加一些关于哪个连接执行的标记：

  /*inner join*/
  (SELECT
     'inner' as join_type 
     , COALESCE(s.name, l.name) as listname
     , AVG(l.overallQuality) AS avgLingQual
     , AVG(s.overallSatisfaction) AS avgSvcQual 
  FROM feedback_linguistic l 
  INNER JOIN feedback_service s ON (l.name = s.name) 
  WHERE (s.createdTime >= '" & date & "' OR s.createdTime IS NULL) 
    AND (l.createdTime >= '" & date & "' OR l.createdTime IS NULL) 
  GROUP BY l.name) 
UNION ALL
  (SELECT
     'left exclusion' as join_type 
     , COALESCE(s.name, l.name) as listname
     , AVG(l.overallQuality) AS avgLingQual
     , AVG(s.overallSatisfaction) AS avgSvcQual 
  FROM feedback_linguistic l 
  LEFT JOIN feedback_service s ON (l.name = s.name) 
  WHERE s.id IS NULL
    /*AND (s.createdTime >= '" & date & "' OR s.createdTime IS NULL) */
    AND (l.createdTime >= '" & date & "' OR l.createdTime IS NULL) 
  GROUP BY l.name) 
UNION ALL
  (SELECT 
     'right exclusion' as join_type
     , COALESCE(s.name, l.name) as listname
     , AVG(l.overallQuality) AS avgLingQual 
     , AVG(s.overallSatisfaction) AS avgSvcQual 
  FROM feedback_linguistic l 
  RIGHT JOIN feedback_service s ON (s.name = l.name) 
  WHERE l.id IS NULL
    AND (s.createdTime >= '" & date & "' OR s.createdTime IS NULL) 
    /*AND (l.createdTime >= '" & date & "' OR l.createdTime IS NULL) */
  GROUP BY s.name) 
ORDER BY listname;

我认为WHERE子句在JOIN之后执行日期过滤，理想情况下我可能会在之前执行此操作。

如果您想先进行过滤，请将其放在join子句中。

在完整外部联接查询中按日期过滤行 - ＆gt;缺少一些结果

背景

问题

2 个答案: