按日期分组，即使没有找到记录，也返回行 [英] Grouping by date, return row even if no records found

问题描述

我有一个查询，用于对表中的所有条目进行分组，并按datetime列进行分组。这一切都很好：

I have a query that groups all entries from a table and groups them by the datetime column. This is all working great:

SELECT SUM(  `value` ) AS  `sum` , DATE(`datetime`) AS  `dt``
FROM  `entry` 
WHERE entryid = 85
AND DATETIME BETWEEN  '2010-01-01' AND '2010-03-01'
GROUP BY `dt`
ORDER BY `datetime`

问题是，我需要它返回即使没有找到任何值，值设置为0。我假设有一些MYSQL函数会照顾这个但似乎找不到。

The problem is, I need it to return a row even if nothing is found, with the value set to 0. I assume there's some MYSQL function that'll take care of this but can't seem to find it.

谢谢！

推荐答案

MySQL没有递归功能，所以你使用NUMBERS表格技巧 -

MySQL doesn't have recursive functionality, so you're left with using the NUMBERS table trick -

1. 创建一个只保存递增数字的表格 - 使用auto_increment方便使用：

1. Create a table that only holds incrementing numbers - easy to do using an auto_increment:

DROP TABLE IF EXISTS `example`.`numbers`;
CREATE TABLE  `example`.`numbers` (
  `id` int(10) unsigned NOT NULL auto_increment,
   PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

使用以下方式填充表：

Populate the table using:

INSERT INTO NUMBERS
  (id)
VALUES
  (NULL)

...您需要的值越多。

...for as many values as you need.

使用 DATE_ADD 构建日期列表，增加日期基于NUMBERS.id值。将2010-01-01和2010-03-01替换为您的开始和结束日期（但使用相同的格式，YYYY-MM-DD） -

Use DATE_ADD to construct a list of dates, increasing the days based on the NUMBERS.id value. Replace "2010-01-01" and "2010-03-01" with your respective start and end dates (but use the same format, YYYY-MM-DD) -

SELECT x.*
  FROM (SELECT DATE_ADD('2010-01-01', INTERVAL n.id - 1 DAY)
          FROM numbers n
         WHERE DATE_ADD('2010-01-01', INTERVAL n.id -1 DAY) <= '2010-03-01' ) x

根据日期时间部分将LEFT JOIN加入到您的数据表上：

LEFT JOIN onto your table of data based on the datetime portion:

   SELECT DATE(x.dt) AS dt,
          COALESCE(SUM(e.value), 0) AS sum_value
     FROM (SELECT DATE_ADD('2010-01-01', INTERVAL n.id - 1 DAY) AS dt
             FROM numbers n
            WHERE DATE_ADD('2010-01-01', INTERVAL n.id -1 DAY) <= '2010-03-01' ) x
LEFT JOIN ENTRY e ON DATE(e.datetime) = x.dt
                 AND e.entryid = 85
 GROUP BY DATE(x.dt) 

为什么数字而不是日期？

简单的日期可以根据数量生成，就像我提供的例子一样。这也意味着使用单个表，而每个数据类型表示一个。

Why Numbers, not Dates?

Simple - dates can be generated based on the number, like in the example I provided. It also means using a single table, vs say one per data type.

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