即使数据丢失，如何从数据库获取按月记录？

即使数据丢失也获取数据库中的按月记录

在数据库管理系统中，通常需要从表中提取按月记录。然而，当某些月份的数据点缺失时，就会出现挑战。本文解决了这个问题，提出了一种解决方案来生成完整的月份列表并检索相应的数据，即使值不存在也是如此。

您提供的原始查询：

SELECT COUNT( `userID` ) AS total, DATE_FORMAT( `userRegistredDate` , '%b' ) AS MONTH , YEAR( `userRegistredDate` ) AS year
FROM `users`
GROUP BY DATE_FORMAT( FROM_UNIXTIME( `userRegistredDate` , '%b' ) )

提供了期望的输出，但仅限于现有数据的几个月。为了处理缺失的月份，我们使用一个包含完整月份列表的表，并将其与用户表连接：

SELECT COUNT(u.userID) AS total, m.month
FROM (
    SELECT 'Jan' AS MONTH
    UNION SELECT 'Feb' AS MONTH
    UNION SELECT 'Mar' AS MONTH
    UNION SELECT 'Apr' AS MONTH
    UNION SELECT 'May' AS MONTH
    UNION SELECT 'Jun' AS MONTH
    UNION SELECT 'Jul' AS MONTH
    UNION SELECT 'Aug' AS MONTH
    UNION SELECT 'Sep' AS MONTH
    UNION SELECT 'Oct' AS MONTH
    UNION SELECT 'Nov' AS MONTH
    UNION SELECT 'Dec' AS MONTH
) AS m
LEFT JOIN users u 
ON MONTH(STR_TO_DATE(CONCAT(m.month, ' 2013'),'%M %Y')) = MONTH(u.userRegistredDate)
AND YEAR(u.userRegistredDate) = '2013'
GROUP BY m.month
ORDER BY 1+1;

此联合表确保所有月份都得到表示，从而获得完整的结果集。下面介绍了一种简化查询的替代方法：

SELECT COUNT(u.userID) AS total, DATE_FORMAT(merge_date,'%b') AS month, YEAR(m.merge_date) AS year
FROM (
    SELECT '2013-01-01' AS merge_date
    UNION SELECT '2013-02-01' AS merge_date
    UNION SELECT '2013-03-01' AS merge_date
    UNION SELECT '2013-04-01' AS merge_date
    UNION SELECT '2013-05-01' AS merge_date
    UNION SELECT '2013-06-01' AS merge_date
    UNION SELECT '2013-07-01' AS merge_date
    UNION SELECT '2013-08-01' AS merge_date
    UNION SELECT '2013-09-01' AS merge_date
    UNION SELECT '2013-10-01' AS merge_date
    UNION SELECT '2013-11-01' AS merge_date
    UNION SELECT '2013-12-01' AS merge_date
) AS m
LEFT JOIN users u 
ON MONTH(m.merge_date) = MONTH(u.userRegistredDate)
AND YEAR(m.merge_date) = YEAR(u.userRegistredDate)
GROUP BY m.merge_date
ORDER BY 1+1;

此修改后的查询使用日期并集来表示完整的月份范围。即使没有所有月份的数据，这两种方法都实现了生成按月记录的目标。

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