如何将函数应用于Postgres中数组列的每个元素？

Question

Pg查询返回一个数组.我想检索每个元素格式化为3位小数.如何将函数应用于数组的每个元素？像下面的东西(显然是错的) -

SELECT Round(ARRAY[1.53224,0.23411234], 2);
{1.532, 0.234}

我想我正在寻找像Perl这样的map功能.

Answer 1

首先,使用unnest将数组转换为集合:

> SELECT n FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
     n      
------------
    1.53224
 0.23411234
(2 rows)

然后,将表达式应用于列:

> SELECT ROUND(n, 2) FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
 round 
-------
  1.53
  0.23
(2 rows)

最后,使用array_agg将集合转换回数组:

> SELECT array_agg(ROUND(n, 2)) FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
  array_agg  
-------------
 {1.53,0.23}
(1 row)

Answer 2

postgres=# select array(select round(unnest(array[1.2,2.4,3,4])));
   array   
-----------
{1,2,3,4}
(1 row)

Answer 3

您可能需要创建存储函数.这是你需要的东西:

CREATE OR REPLACE FUNCTION array_round(float[], int)
RETURNS float[]
AS
$$
DECLARE
   arrFloats ALIAS FOR $1;
   roundParam ALIAS FOR $2;
   retVal float[];
BEGIN
   FOR I IN array_lower(arrFloats, 1)..array_upper(arrFloats, 1) LOOP
    retVal[I] := round(CAST(arrFloats[I] as numeric), roundParam);
   END LOOP;
RETURN retVal;
END;
$$
LANGUAGE plpgsql 
   STABLE 
RETURNS NULL ON NULL INPUT;

然后调用这样的东西:

# SELECT array_round(ARRAY[1.53224,0.23411234], 2);
 array_round 
-------------
 {1.53,0.23}
(1 row)