Question

我快要把头发拉过这个了。这是我正在使用的查询：

           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，我得到101个结果。如果我在查询的末尾添加一个简单的LIMIT 20，那么查询会突然返回一个结果。

好像这不够奇怪，如果我将LIMIT 20更改为LIMIT 25，我会收到25个结果。有什么神奇的东西我对此缺失了吗？我不能为我的生活弄明白。

P.S。 - 我尝试过SQL_NO_CACHE无济于事。 P.P.S. - 如果我删除整个ORDER BY子句，但是保留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......你已经离开了我不相信你真正想要的联接...只是NORMAL加入...我会先尝试一下解析您的查询并查看它可能会显示有关索引，全表扫描等的洞察力......

此外，使用select *通常是糟糕且懒惰的查询。获取您想要和期望的列。如果一个结构发生了变化并且你依赖于*，那么将来你可能无法预见到它。

我将重组如下，查看结果并检查ITs 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

查看你的IF（）条款的质量，评论，轻松......你是否故意仅在所有3个元素的p.Reviews = 0时获取这些值...或者它们是否应该代表值的每一列...

这是一个MySQL错误还是我疯了？查询未返回足够的结果

1 个答案: