postgresql - 使用两列作为行数据来透视 PostgreSQL 中的数据

Question

在 PostgreSQL 中，我有一个表 country 和一个表 property。一个国家可以拥有很多属性(property)。每个属性都有一个 created_at。我需要按年和周计算每个国家/地区的属性。举个例子，我有一个国家的查询。

SELECT 
  EXTRACT(YEAR FROM created_at) AS yy, 
  EXTRACT(WEEK FROM created_at) AS week, 
  COUNT(id) AS country_1
FROM property 
WHERE id_country = 1
GROUP BY week, yy 
ORDER BY yy, week ASC

结果是这样的

yy  |week|country_1
---------------
2014|1   |5
2014|2   |1154
2014|3   |769
...

所以，我需要这样的结果

yy  |week|country_1|country_2|country_3
----------------------------------------
2014|1   |5        |56       |543
2014|2   |1154     |1234     |432
2014|3   |769      |123      |432
...

这可能吗？查询会怎样？

Answer 1

没有任何扩展的替代解决方案:

SELECT 
  EXTRACT(YEAR FROM created_at) AS yy, 
  EXTRACT(WEEK FROM created_at) AS week, 
  COUNT(CASE WHEN id_country = 1 THEN id ELSE null END) AS country_1,
  COUNT(CASE WHEN id_country = 2 THEN id ELSE null END) AS country_2,
  COUNT(CASE WHEN id_country = 3 THEN id ELSE null END) AS country_3,
  ...
  COUNT(CASE WHEN id_country = 7 THEN id ELSE null END) AS country_7,
FROM property 
GROUP BY week, yy 
ORDER BY yy, week ASC;

如果您使用的是 pg 9.4 或更高版本，您可以更换

COUNT(CASE WHEN id_country = 2 THEN id ELSE null END) AS country_2

与

COUNT(id) FILTER (WHERE id_country = 2) AS country_2

等等。