Question

关注问题＆＃34; Select new or returning items for a specified year＆＃34;，我现在想要创建有关每年新增或返回的项目数的统计信息。这是我的示例表：

+---------------------+----------+
| date                | item     |
+---------------------+----------+
| 2008-11-30 11:15:59 | Plums    |
| 2012-11-08 19:42:37 | Lemons   |
| 2013-01-30 18:58:07 | Apples   |
| 2013-02-12 13:44:45 | Pears    |
| 2014-06-08 11:46:48 | Apples   |
| 2014-09-01 20:28:03 | Oranges  |
+---------------------+----------+

我正在寻找一个查询，该查询将返回之前年未出现的项目数量，以及前几年已经出现的项目数量。

结果应该看起来像这样：

Year   New Items   Returning Items
2008   1           0
2012   1           0
2013   2           0
2014   1           1

如何实现这一目标？

Answer 1

好吧，使用您要链接的查询，我想出了以下内容（SQL Fiddle）：

SELECT m1.UniqYear,  
  CASE WHEN m2.NewItems IS NULL THEN 0 ELSE m2.NewItems END AS NewItems,
  CASE WHEN m3.ReturningItems IS NULL THEN 0 ELSE m3.ReturningItems END AS ReturningItems
FROM 
(
  SELECT DISTINCT YEAR(s.date) AS UniqYear 
  FROM MyTable s
) m1 LEFT JOIN 
(
  SELECT YEAR(s1.date) AS Year, COUNT(*) AS NewItems
  FROM MyTable s1
    LEFT JOIN MyTable s2 ON s1.item = s2.item AND YEAR(s2.date) < YEAR(s1.date)
  WHERE s2.date IS NULL
  GROUP BY YEAR(s1.date)
) m2 ON m1.UniqYear = m2.Year
LEFT JOIN 
(
  SELECT YEAR(s3.date) AS Year, COUNT(*) AS ReturningItems
  FROM MyTable s3
    INNER JOIN MyTable s4 ON s3.item = s4.item
  WHERE YEAR(s4.date) < YEAR(s3.date)
  GROUP BY YEAR(s3.date)
) m3 ON m1.UniqYear = m3.Year
ORDER BY m1.UniqYear;

我使用 CASE 语句返回0而不是null。第一个子查询（m1）用于获取不同的年份。第二个子查询（m2）用于获取新项计数。第三个子查询（m3）用于获取返回项计数。请注意，我必须添加group by语句以获得每年的计数。我必须LEFT JOIN ed m2和m3到m1，因为您并不总是会有 NewItems 计数或返回每年的项目。

按年计算新项目和返回项目

1 个答案: