Question

我在Postgres（9.2）中有一个表格如下：

id | code | year
 1 |  A   | 2014
 1 |  A   | 2015
 1 |  A   | 2016
 2 |  A   | 2014
 2 |  A   | 2015
 2 |  B   | 2016

我正在尝试根据id字段组合行。我可以通过自联接来实现，但是我试图避免在可能的情况下扫描表两次（可能是数百万行）。我想做这样的事情：

SELECT CASE year WHEN 2016 THEN code ELSE '' END AS code,
       CASE year WHEN 2015 THEN code ELSE '' END AS prev_code
FROM tbl
GROUP BY id
HAVING year = 2015 OR year = 2016

理想情况下，输出看起来像这样：

code | prev_code
  A  |    A         (from id=1)
  B  |    A         (from id=2)

当我运行我提出的查询时，它告诉我year需要在group by或一个聚合函数中。在互联网上有类似查询的例子，但它们似乎不适用于Postgres。知道如何在Postgres 9.2中做到这一点吗？

Answer 1

你可以使用Lag window function：

Select Z.Code, Z.PrevCode
From (
     Select Id, Code, Year
        , lag( Code, 1 ) Over ( Partition By Id Order By Year ) As PrevCode
     From tbl
     Where Year In(2016,2015)
     ) As Z
Where Z.Year = 2016

SQL Fiddle version

Answer 2

您可以按如下方式稍微修改您的查询：

SELECT MAX(CASE year WHEN 2016 THEN code ELSE '' END) AS code,
       MAX(CASE year WHEN 2015 THEN code ELSE '' END) AS prev_code
FROM Table1 tbl
WHERE year IN(2015, 2016)
GROUP BY id;

SQL Fiddle

从Postgres中的分组行中选择值

2 个答案: