在 PostgreSQL 中旋转 table

Question

请帮我一个研究案例如下：我在 Postgres 中有一个 table：

以及如何从 Table A 创建视图或 Table B，如下所示：

我尝试使用交叉表，但它不起作用。

Error

非常感谢！

Answer 1

您可以使用 row_number() 和条件聚合：

select item_id,
       max(author) filter (where seqnum = 1) as author_1,
       max(author) filter (where seqnum = 2) as author_2,
       max(author) filter (where seqnum = 3) as author_3,
       max(author) filter (where seqnum = 4) as author_4
from (select a.*,
             row_number() over (partition by item_id order by author) as seqnum
      from tablea a
     ) a
group by item_id;

Answer 2

我会将作者聚合到一个数组中，然后 select 外部查询中的数组元素：

select item_id, 
       authors[1] as author_1,
       authors[2] as author_2,
       authors[3] as author_3
from (
  select item_id, 
         array_agg(author order by author) as authors
  from items
  group by item_id
) t;

在 PostgreSQL 中旋转 table

Rotate table in PostgreSQL

sql

postgresql

pivot