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背景

我有两张表，在 MySQL 中有不同类型的反馈项。我已经构建了一个查询来组合这些表FULL OUTER JOIN（实际上在 MySQL 中写为两个连接和一个联合）并计算一些平均成绩。这个查询似乎完美地工作：

  (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，但在这里没有什么区别）

问题

接下来我尝试添加按日期过滤（即只考虑在某个日期之后创建的反馈项）。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;

这几乎可行：我得到的结果看起来是正确的。但是，缺少一些反馈项。例如，设置一个月前的日期，我在数据库中统计了 21 个人的反馈，但这个查询只返回 19 个人。最糟糕的是，我似乎找不到丢失的物品之间的任何相似之处。

我在这个查询中做错了吗？我认为该WHERE子句在 the 之后进行日期过滤JOIN，理想情况下我可能会在此之前进行。再说一次，我不知道这是否会导致我的问题，而且我也不知道如何以不同的方式编写此查询。

4

2 回答 2

2

我接受了约翰斯的回答，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，这个答案也很有用。但是，我想我也会发布我到达的第一个解决方案。它正在使用子查询：

  (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，该解决方案看起来并不理想，因为根据我的经验，子查询有时很慢。再说一次，我没有做任何性能分析，所以这里使用子查询可能不是瓶颈。无论如何，它在我的应用程序中运行得足够快。

于 2011-10-29T08:48:46.810 回答
1

完全外连接是 3 个连接的组合：

1- A 和 B 之间的内连接
2- A 和 B 之间的左排除连接
3- A 和 B 之间的右排除连接

请注意，内部和左排除联接的组合是左外部联接，因此您通常将查询重写为left outer join+ right exclusion join。
然而，出于调试目的，它对union所有 3 个连接都很有用，并添加一些关于哪个连接做什么的标记：

  /*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 之后进行日期过滤，理想情况下我可能会在此之前这样做。

如果你想在之前做过滤，那么把它放在join子句中。

于 2011-10-27T11:56:17.447 回答