如何 select 整数和数组元素？

Question

我能够 select 将 "col3" 中的重复值放入一个数组中，并使用以下方法按 "col1"、"col2" 排列它们：

select "col1", "col2", array_agg("col3"  order by "col1", "col2") as myArray FROM 
     myTable group by "col1", "col2"

接下来，我想 select 数组的前两个元素（因此它们出现在自己的列中）并继续 "col1"、"col2" 排列它们像这样：

select "col1", "col2", element[1], element[2] from 
    (select "col1", "col2", array_agg("col3"  order by "col1", "col2") as myArray FROM 
         myTable group by "col1", "col2") as  vv(element)

但是，我收到以下错误：

ERROR:  cannot subscript type integer because it is not an array

有没有办法转换 "col1" 和 "col2" 的整数值，以便我可以将数组元素放入由 "col1" 和 "col2" 组织的单独列中？

Answer 1

您的 table 别名 vv(element) 是错误的，因为内部查询有 3 列，而不仅仅是一列。通过仅指定一列，您仅将第一列重命名（col1 至 element），而其他两列在内部查询中保留其原始名称。

因此您的别名需要为所有三列提供名称：

select "col1", "col2", element[1], element[2] 
from (
   select "col1", "col2", array_agg("col3"  order by "col1", "col2") as myArray 
   FROM myTable 
   group by "col1", "col2"
) as vv(co1, col2, element);

vv(co1, col2, element) 本质上是将 myarray 重命名为 element.

由于内部查询中的所有列都有专有名称，因此您实际上不需要 table 别名中的列别名：

select "col1", "col2", myarray[1] as element_one, myarray[2]  as element_two
from (
   select "col1", "col2", array_agg("col3"  order by "col1", "col2") as myArray 
   FROM myTable 
   group by "col1", "col2"
) as vv;

如何 select 整数和数组元素？

How can I select integers and array elements?

sql

arrays

postgresql

alias