arrays - 如何在 Postgres 中选择数组的子集？

Question

假设我的一个 postgres 列中有一个数组:

> select array[1,2,3,4];
   array   
-----------
 {1,2,3,4} 

通常如何从该数组中选择项目的子集？例如，如果我想从 x 索引到 y 索引中选择项目，我将如何获取该范围内的项目 (x到 y)?

我正在使用 PostgreSQL 9.4。

Answer 1

来自fine manual :

8.15.3. Accessing Arrays
[...]
We can also access arbitrary rectangular slices of an array, or subarrays. An array slice is denoted by writing lower-bound:upper-bound for one or more array dimensions.
[...]
It is possible to omit the lower-bound and/or upper-bound of a slice specifier; the missing bound is replaced by the lower or upper limit of the array's subscripts.

例如:

=> select (array[1,2,3,4,5,6])[2:5];
   array   
-----------
 {2,3,4,5}
(1 row)

=> select (array[1,2,3,4,5,6])[:5];
    array    
-------------
 {1,2,3,4,5}
(1 row)

=> select (array[1,2,3,4,5,6])[2:];
    array    
-------------
 {2,3,4,5,6}
(1 row)

=> select (array[1,2,3,4,5,6])[:];
     array     
---------------
 {1,2,3,4,5,6}
(1 row)

因此，要获取从索引 x 到 y(含)的切片，您会说:

array_column[x:y]