普雷斯托中的压缩数组

Question

我有一个查询，使用它们array_agg()函数生成数组字符串

SELECT 
array_agg(message) as sequence
from mytable
group by id

这将生成如下所示的表：

                 sequence
1 foo foo bar baz bar baz
2     foo bar bar bar baz
3 foo foo foo bar bar baz

但我的目标是压缩字符串数组，以便没有字符串可以连续重复多次，例如，所需的输出如下所示：

    sequence
1 foo bar baz bar baz
2 foo bar baz
3 foo bar baz

如何使用Presto SQL来做到这一点？

Answer 1

您可以通过以下两种方式之一执行此操作：

1. 使用 array_distinct 函数从生成的数组中删除重复项：

WITH mytable(id, message) AS (VALUES
  (1, 'foo'), (1, 'foo'), (1, 'bar'), (1, 'bar'), (1, 'baz'), (1, 'baz'),
  (2, 'foo'), (2, 'bar'), (2, 'bar'), (2, 'bar'), (2, 'baz'),
  (3, 'foo'), (3, 'foo'), (3, 'foo'), (3, 'bar'), (3, 'bar'), (3, 'baz')
)
SELECT array_distinct(array_agg(message)) AS sequence
FROM mytable
GROUP BY id

2. 使用聚合中的 DISTINCT 限定符在将重复值传递到array_agg之前将其删除。

WITH mytable(id, message) AS (VALUES
  (1, 'foo'), (1, 'foo'), (1, 'bar'), (1, 'bar'), (1, 'baz'), (1, 'baz'),
  (2, 'foo'), (2, 'bar'), (2, 'bar'), (2, 'bar'), (2, 'baz'), (3, 'foo'),
  (3, 'foo'), (3, 'foo'), (3, 'bar'), (3, 'bar'), (3, 'baz')
)
SELECT array_agg(DISTINCT message) AS sequence
FROM mytable
GROUP BY id

两种备选方案产生相同的结果：

    sequence
-----------------
 [foo, bar, baz]
 [foo, bar, baz]
 [foo, bar, baz]
(3 rows)

更新：您可以使用最近引入的MATCH_RECOGNIZE功能删除重复的元素序列：

WITH mytable(id, message) AS (VALUES
  (1, 'foo'), (1, 'foo'), (1, 'bar'), (1, 'baz'), (1, 'bar'), (1, 'baz'),
  (2, 'foo'), (2, 'bar'), (2, 'bar'), (2, 'bar'), (2, 'baz'),
  (3, 'foo'), (3, 'foo'), (3, 'foo'), (3, 'bar'), (3, 'bar'), (3, 'baz')
)
SELECT array_agg(value) AS sequence
FROM mytable
 MATCH_RECOGNIZE(
    PARTITION BY id
    MEASURES A.message AS value
    PATTERN (A B*)
    DEFINE B AS message = PREV(message)
)
GROUP BY id