这是一个MySQL错误还是我疯了？查询没有返回足够的结果

Question

我靠近这一点拉我的头发。这是我正在处理的查询：

  SELECT 
      *, 
      GROUP_CONCAT(DISTINCT d.title SEPARATOR ', ') AS departments, 
      MAX(d.department_id) AS department_id, 
      CONCAT(friendly_last_name, ', ', friendly_first_name) AS professor_name, 
      IF(p.reviews = 0, "", p.quality) AS quality, 
      IF(p.reviews = 0, "", p.reviews) AS reviews, 
      IF(p.reviews = 0, "", p.ease) AS ease, 
      u.url 
     FROM 
      educational.professors_departments 
      LEFT JOIN educational.professors p USING (professor_id) 
      LEFT JOIN educational.departments d USING (department_id) 
      LEFT JOIN educational.universities u ON (p.university_id = u.university_id) 
     WHERE 
      p.university_id = 3231 
      AND d.department_id IN ('91') 
     GROUP BY 
      professor_id 
     ORDER BY p.friendly_last_name ASC, p.friendly_first_name ASC 

当我按原样运行该查询时，得到101个结果。如果我在查询的末尾添加一个简单的LIMIT 20，则突然查询返回一个结果。

好像这并不奇怪，如果我将LIMIT 20更改为LIMIT 25，我会收到25个结果。是否有一些神奇的东西，我错过了这个？我不能为我的生活弄清楚。

P.S. - 我试过SQL_NO_CACHE无济于事。 P.P.S. - 如果我删除整个ORDER BY子句，但保留LIMIT 20 ...我得到20个结果

Answer 1

我只能猜测引擎正在尽力猜测一些元素...您已经离开了连接不相信是你真正想要的......只是NORMAL加入...我首先试着对你的查询进行一次EXPLAIN，看看它出现了什么，可能会显示与索引，全表扫描等的洞察力...

此外，使用select *通常是不好的，懒惰的查询。获取你想要和期望的列。如果一个结构发生了变化，而你依靠*，那么你将来可能无法预见到结果。

我会重组如下，看到的结果并检查其EXPLAIN

SELECT STRAIGHT_JOIN 
     p.Professor_ID, 
     CONCAT(p.friendly_last_name, ', ', p.friendly_first_name) AS professor_name, 
     GROUP_CONCAT(DISTINCT d.title SEPARATOR ', ') AS departments, 
     MAX(d.department_id) AS department_id, 
     IF(p.reviews = 0, "", p.quality) AS quality, 
     IF(p.reviews = 0, "", p.reviews) AS reviews, 
     IF(p.reviews = 0, "", p.ease) AS ease, 
     u.url 
    FROM 
     educational.professors p 
     JOIN educational.professors_departments pd 
      on p.Professor_ID = pd.Professor_ID 
      JOIN educational.departments d 
       on pd.Department_ID = d.Department_ID 
       AND d.department_id IN ('91') 
     JOIN educational.universities u 
      ON p.university_id = u.university_id 
    WHERE 
     p.university_id = 3231 
    GROUP BY 
     p.professor_id 
    ORDER BY 
     p.friendly_last_name ASC, 
     p.friendly_first_name ASC 
    LIMIT 20 

看你的IF（）子句质量，评价，缓解......是你故意只在P得到这些值仅。所有3个元素的评论= 0 ...或者他们应该代表每个值的列...