MySQL 查询优化 - 分组值答案

【问题标题】：MySQL Query Optimization - Grouping ValuesMySQL 查询优化 - 分组值
【发布时间】：2016-02-06 05:16:42
【问题描述】：

好的，这是我的数据库结构：

TABLE `books`
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT
  `title` varchar(25) COLLATE utf8_unicode_ci NOT NULL DEFAULT ''
  `author` varchar(25) COLLATE utf8_unicode_ci NOT NULL DEFAULT ''
  PRIMARY KEY (`id`)
  UNIQUE KEY `title` (`title`)

TABLE `chapters`
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT
  `book_id` smallint(5) unsigned NOT NULL DEFAULT '0'
  `number` smallint(5) unsigned NOT NULL DEFAULT '0'
  `title` varchar(50) COLLATE utf8_unicode_ci NOT NULL DEFAULT ''
  PRIMARY KEY (`id`)
  KEY `book_id` (`book_id`) -> Foreign on `books`.`id`

TABLE `pages`
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT
  `book_id` smallint(5) unsigned NOT NULL DEFAULT '0'
  `chapter_id` smallint(5) unsigned NOT NULL DEFAULT '0'
  `number` smallint(5) unsigned NOT NULL DEFAULT '0' // real number, for page 1 of book 1 this has value 1, for page 1 of book 2 this has value 1...
  `value` float NOT NULL DEFAULT '0'
  PRIMARY KEY (`id`)
  KEY `book_id` (`book_id`) -> Foreign on `books`.`id`
  KEY `chapter_id` (`chapter_id`)  -> Foreign on `chapters`.`id`

现在...在 PHP 页面上，我想显示每本书的平均 pages.value、每本书每章的平均 pages.value 以及每一页的单个 pages.value：

Book 1 AVG = X
Book 1 Chapter   1            AVG = X
Book 1 Chapter ...            AVG = X
Book 1 Chapter   N            AVG = X
Book 1 Chapter   1 Page   1   VAL = X
Book 1 Chapter   1 Page ...   VAL = X
Book 1 Chapter   1 Page   N   VAL = X

And so on...

当然，使用 3 个查询和一些连接会非常简单，我可以使用 PHP 以编程方式重新格式化数据以满足我的需求。但我想知道是否可以将这些查询（顺便说一句，执行起来很慢）优化为一个查询，或者通过使用高级语法来减少处理时间。

【问题讨论】：

GROUP BY WITH ROLLUP 大概会给你你想要的 - 或者你可以在 PHP 中进行最终计算。想必所有页面都属于一个章节？
Value 是包含浮点数的列。没关系。页面属于多个章节。
@Strawberry 。 . .唉，GROUP BY WITH ROLLUP 不会正确计算平均值。

标签： php jquery mysql sql

【解决方案1】：

您希望用于搜索的索引列有很大帮助。例如：

CREATE INDEX index_name ON table_name(column_name);

完成同样事情的另一种方法是：

ALTER TABLE table_name ADD INDEX ( column_name );

这样你的查询执行时间会很快...

【讨论】：

【解决方案2】：

一种方法是使用ROLLUP，但正如 Gordon 指出的那样，这可能不会返回您期望的平均值。这取决于您如何定义这些聚合。但是，我怀疑 MySQL 确实以您期望的方式定义了这些平均值。这是一个例子，它简洁地说明了如何回答一个问题以及如何提出一个问题（并且可能识别出这种方法中的缺陷，其他人可以批评）......

CREATE TABLE pages
(page_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,book VARCHAR(20) NOT NULL
,chapter_no INT NOT NULL
,page_no INT NOT NULL
,words INT NOT NULL
,UNIQUE(book,chapter_no,page_no)
);

INSERT INTO pages (book,chapter_no,page_no,words) VALUES
('The Hobbit',1,1,50),
('The Hobbit',1,2,60),
('The Hobbit',1,3,100),
('The Hobbit',1,4,40),
('The Hobbit',2,1,60),
('The Hobbit',2,2,40),
('The Hobbit',2,3,20),
('Oliver Twist',1,1,120),
('Oliver Twist',1,2,30),
('Oliver Twist',1,3,30),
('Oliver Twist',2,1,100),
('Oliver Twist',2,2,50);

SELECT * 
  FROM pages 
 ORDER  
    BY book
      ,chapter_no
      , page_no;
+---------+--------------+------------+---------+-------+
| page_id | book         | chapter_no | page_no | words |
+---------+--------------+------------+---------+-------+
|       8 | Oliver Twist |          1 |       1 |   120 |
|       9 | Oliver Twist |          1 |       2 |    30 |
|      10 | Oliver Twist |          1 |       3 |    30 |
|      11 | Oliver Twist |          2 |       1 |   100 |
|      12 | Oliver Twist |          2 |       2 |    50 |
|       1 | The Hobbit   |          1 |       1 |    50 |
|       2 | The Hobbit   |          1 |       2 |    60 |
|       3 | The Hobbit   |          1 |       3 |   100 |
|       4 | The Hobbit   |          1 |       4 |    40 |
|       5 | The Hobbit   |          2 |       1 |    60 |
|       6 | The Hobbit   |          2 |       2 |    40 |
|       7 | The Hobbit   |          2 |       3 |    20 |
+---------+--------------+------------+---------+-------+

SELECT book
     , chapter_no
     , page_no
     , AVG(words) words_per_page 
  FROM pages 
 GROUP 
    BY book
     , chapter_no
     , page_no WITH ROLLUP;
+--------------+------------+---------+----------------+
| book         | chapter_no | page_no | words_per_page |
+--------------+------------+---------+----------------+
| Oliver Twist |          1 |       1 |       120.0000 |
| Oliver Twist |          1 |       2 |        30.0000 |
| Oliver Twist |          1 |       3 |        30.0000 |
| Oliver Twist |          1 |    NULL |        60.0000 |<-- avg words per page chapter 1
| Oliver Twist |          2 |       1 |       100.0000 |
| Oliver Twist |          2 |       2 |        50.0000 |
| Oliver Twist |          2 |    NULL |        75.0000 |<-- avg words per page chapter 2
| Oliver Twist |       NULL |    NULL |        66.0000 |<-- avg words per page entire book
| The Hobbit   |          1 |       1 |        50.0000 |
| The Hobbit   |          1 |       2 |        60.0000 |
| The Hobbit   |          1 |       3 |       100.0000 |
| The Hobbit   |          1 |       4 |        40.0000 |
| The Hobbit   |          1 |    NULL |        62.5000 |<-- avg words per page chapter 1
| The Hobbit   |          2 |       1 |        60.0000 |
| The Hobbit   |          2 |       2 |        40.0000 |
| The Hobbit   |          2 |       3 |        20.0000 |
| The Hobbit   |          2 |    NULL |        40.0000 |<-- avg words per page chapter 2
| The Hobbit   |       NULL |    NULL |        52.8571 |<-- avg words per page entire book
| NULL         |       NULL |    NULL |        58.3333 |<-- avg words per page all books
+--------------+------------+---------+----------------+

【讨论】：