如何从二维数组中选择一维数组? [英] How to select 1d array from 2d array?
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问题描述
我有二维数组,只想选择其中的第一个元素,即一维数组.
我该怎么做?
I have 2d array and want to select only first element of it, which is 1d array.
How do I do that?
推荐答案
获取数组:
To get the first slice of an array:
SELECT my_arr[1:1];
结果数组与输入具有相同的数组维度.
我之前的回答中的详细信息:
The resulting array has the same array dimensions as the input.
Details in my previous answer here:
要展平结果:
SELECT ARRAY(SELECT unnest(my_arr[1:1]));
或者更清洁:
SELECT ARRAY(SELECT * FROM unnest(my_arr)[1:1]));
示例
SELECT (ARRAY[[1,2,3], [4,5,6], [7,8,9]])[1:1];
结果:
{{1,2,3}} -- 2D array
或者:
SELECT ARRAY(
SELECT unnest((ARRAY[[1,2,3], [4,5,6], [7,8,9]])[1:1])
);
结果:
{1,2,3} -- 1D array
在 Postgres 8.3 中模拟 unnest()
回复您的评论:
您链接到的 Wiki 页面 有点误导.我用二维数组的代码更新了它.
Emulate unnest()
in Postgres 8.3
Response to your comment:
The Wiki page you are linking to was a bit misleading. I updated it with code for 2-dimensional arrays.
unnest()
用于一维数组:
CREATE OR REPLACE FUNCTION unnest_1d(anyarray)
RETURNS SETOF anyelement AS
$func$
SELECT $1[i]
FROM generate_series(array_lower($1,1), array_upper($1,1)) i
$func$ LANGUAGE sql IMMUTABLE;
unnest()
二维数组:
CREATE OR REPLACE FUNCTION unnest_2d(anyarray)
RETURNS SETOF anyelement AS
$func$
SELECT $1[d1][d2]
FROM generate_series(array_lower($1,1), array_upper($1,1)) d1
, generate_series(array_lower($1,2), array_upper($1,2)) d2
$func$ LANGUAGE sql IMMUTABLE;
聚合函数 array_agg()
在 Postgres 8.3 中默认没有安装:
The aggregate function array_agg()
is not installed by default in Postgres 8.3:
CREATE AGGREGATE array_agg(anyelement) (
SFUNC = array_append,
STYPE = anyarray,
INITCOND = '{}'
);
将二维数组取消嵌套到一维数组:
Unnest 2d array to 1d arrays:
CREATE OR REPLACE FUNCTION unnest_2d_1d(anyarray)
RETURNS SETOF anyarray AS
$func$
SELECT array_agg($1[d1][d2])
FROM generate_series(array_lower($1,1), array_upper($1,1)) d1
, generate_series(array_lower($1,2), array_upper($1,2)) d2
GROUP BY d1
ORDER BY d1
$func$ LANGUAGE sql IMMUTABLE;
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