如何使用GROUP_CONCAT和/或NEST但限制結果中元素的數量

Question

我經常聚合用戶表的度量標準，其中每個聚合組存儲桶的大小爲數以萬計。我希望能夠爲每個存儲桶提供幾個示例用戶，但不是長度爲數千或可能爲數百萬的GROUP_CONCAT。

Answer 1

這是不可能與傳統的SQL聚合函數，但它是可能的standard SQL及其聚合函數STRING_AGG（相當於GROUP_CONCAT在傳統SQL）和ARRAY_AGG（相當於NEST）。這兩種功能均支持「Using LIMIT with aggregate functions」中所述的可選LIMIT子句。

例如：

select string_agg(x LIMIT 2) 
from unnest(['hello', 'world!', 'foo', 'bar', 'baz']) x 

返回'hello,world!'串，和

select array_agg(x LIMIT 2) 
from unnest(['hello', 'world!', 'foo', 'bar', 'baz']) x 

['hello', 'world!']返回陣列。

Answer 2

對於Legacy SQL

SELECT GROUP_CONCAT_UNQUOTED(x) FROM (
    SELECT x FROM 
    (SELECT 'hello' AS x), 
    (SELECT 'world!' AS x), 
    (SELECT 'foo' AS x), 
    (SELECT 'bar' AS x), 
    (SELECT 'baz' AS x), 
LIMIT 2 
) 

和

SELECT NEST(x) FROM (
    SELECT x FROM 
    (SELECT 'hello' AS x), 
    (SELECT 'world!' AS x), 
    (SELECT 'foo' AS x), 
    (SELECT 'bar' AS x), 
    (SELECT 'baz' AS x), 
LIMIT 2 
) 

它們分別返回'hello,world!'串和['hello', 'world!'] 「陣列」

下面

是例如用GROUP BY

SELECT id, GROUP_CONCAT_UNQUOTED(x) FROM (
    SELECT id, x, ROW_NUMBER() OVER(PARTITION BY id) AS num FROM 
    (SELECT 1 AS id, 'hello' AS x), 
    (SELECT 1 AS id, 'world!' AS x), 
    (SELECT 1 AS id, 'foo' AS x), 
    (SELECT 1 AS id, 'bar' AS x), 
    (SELECT 1 AS id, 'baz' AS x), 
    (SELECT 2 AS id, 'hello2' AS x), 
    (SELECT 2 AS id, 'world2!' AS x), 
    (SELECT 2 AS id, 'foo2' AS x), 
    (SELECT 2 AS id, 'bar2' AS x), 
    (SELECT 2 AS id, 'baz2' AS x), 
) 
WHERE num < 3 
GROUP BY id