在分组/聚合期间合并/合并数组值 [英] Concatenate/merge array values during grouping/aggregation

问题描述

我有一个表，该表的数组列类型为：

I have a table with the an array column type:

 title       tags
"ridealong";"{comedy,other}"
"ridealong";"{comedy,tragedy}"
"freddyjason";"{horror,silliness}"

我想编写一个查询，为每个标题生成单个数组（理想情况下，它将是一个设置/去重复的数组）

I would like to write a query that produces a single array per title(in an ideal world it would be a set/deduplicated array)

eg

select array_cat(tags),title from my_test group by title

上面的查询当然行不通，但是我想产生两行：

The above query doesn't work of course, but I would like to produce 2 rows:

"ridealong";"{comedy,other,tragedy}"
"freddyjason";"{horror,silliness}"

任何帮助或指针都将非常感激
（我使用的是Postgres 9.1）

Any help or pointers would be very much appreciated (I am using Postgres 9.1)

基于Craig的帮助，我得到了以下内容（语法略有更改，因为9.1完全按照他的显示抱怨该查询）

Based on Craig's help I ended up with the following (slightly altered syntax since 9.1 complains about the query exactly as he shows it)

SELECT t1.title, array_agg(DISTINCT tag.tag) 
FROM my_test t1, (select unnest(tags) as tag,title from my_test) as tag 
where tag.title=t1.title
GROUP BY t1.title;

推荐答案

自定义汇总

方法1：定义自定义汇总。 这是我之前写过的书。

CREATE TABLE my_test(title text, tags text[]);

INSERT INTO my_test(title, tags) VALUES
('ridealong', '{comedy,other}'),
('ridealong', '{comedy,tragedy}'),
('freddyjason', '{horror,silliness}');

CREATE AGGREGATE array_cat_agg(anyarray) (
  SFUNC=array_cat,
  STYPE=anyarray
);

select title, array_cat_agg(tags) from my_test group by title;

横向查询

...或由于您不想保留订单并希望删除重复数据，因此可以使用 LATERAL 查询，例如：

SELECT title, array_agg(DISTINCT tag ORDER BY tag) 
FROM my_test, unnest(tags) tag 
GROUP BY title;

在这种情况下，您不需要自定义汇总。由于重复数据删除，对于大数据集而言，这可能会稍微慢一些。不过，如果不需要，可以删除 ORDER BY 。

in which case you don't need the custom aggregate. This one is probably a fair bit slower for big data sets due to the deduplication. Removing the ORDER BY if not required may help, though.

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