sql - PostgreSQL Group By A，但仍拉取B字段

Question

我有这样的表:

rpt_date   | shipping_id | total_usage 
--------------------------------------
2013-11-01 | 1           | 10
2013-11-01 | 2           | 2
2013-11-01 | 3           | 5
2013-11-02 | 1           | 15
2013-11-02 | 4           | 1
2013-11-03 | 1           | 20
2013-11-03 | 2           | 3

我想使用单个查询来获得这样的结果:

rpt_date   | 1 (shipping _id) | 2 (shipping_id) | 3 (shipping_id) | 4 (shipping_id)
-----------------------------------------------------------------------------------
2013-11-01 | 10 (total_usage) | 2               | 5               | 0
2013-11-02 | 15               | 0               | 0               | 1
2013-11-03 | 20               | 3               | 0               | 0

我该如何查询呢？我正在尝试这样查询

SELECT 
  rpt_date,
  CASE WHEN shipping_id = 1 THEN total_usage ELSE 0 END AS a,
  CASE WHEN shipping_id = 2 THEN total_usage ELSE 0 END AS b,
  CASE WHEN shipping_id = 3 THEN total_usage ELSE 0 END AS c,
  CASE WHEN shipping_id = 4 THEN total_usage ELSE 0 END AS d
FROM 
  Table A
WHERE 
  DATE(rpt_date) BETWEEN '2013-11-01' AND '2013-11-03' 
GROUP BY 
  rpt_date 
ORDER BY 
  rpt_date

上面的查询提示错误，shipping_id 和 total_usage 必须是 GROUP BY 的一部分。我的查询有什么问题？

Answer 1

用 SUM 试试:

SELECT 
  rpt_date,
  SUM(CASE WHEN shipping_id = 1 THEN total_usage ELSE 0 END) AS a,
  SUM(CASE WHEN shipping_id = 2 THEN total_usage ELSE 0 END) AS b,
  SUM(CASE WHEN shipping_id = 3 THEN total_usage ELSE 0 END) AS c,
  SUM(CASE WHEN shipping_id = 4 THEN total_usage ELSE 0 END) AS d
FROM TableA
WHERE DATE(rpt_date) BETWEEN '2013-11-01' AND '2013-11-03' 
GROUP BY rpt_date 
ORDER BY rpt_date

sqlfiddle demo

您正在按 rpt_date 分组，但没有告诉它其他列应该发生什么。通过这种方式，您可以获得这些日期的 total_usage 的总和。如果您只有一条记录，对于每个日期和每个 shipping_id，它将只返回该值。