Presto - 联合和限制答案

【问题标题】：Presto - Union & LimitPresto - 联合和限制
【发布时间】：2021-03-08 11:59:14
【问题描述】：

目前，我有一个 CTE，它正在构建一个数据列表和一个随机行号

我想要做的是根据一些标准输出几个联合在一起的查询。查询可以与联合一起正常工作，但当我为任何查询添加限制时将无法正常工作。

有没有一种方法可以运行查询并获取不同的子集？

例子：

with selection as (
select account, address, type, random(1000)
from details
)

select 
  account,
  address
from details
where type = 'a'
order by random 
limit 50

union all

select 
  account,
  address
from details
where type = 'b'
order by random 
limit 50

union all

select 
  account,
  address
from details
where type = 'c'
order by random 
limit 50

【问题讨论】：

请注意，selection 未使用。

标签： sql union presto

【解决方案1】：

其实这里根本不需要联合：

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY type ORDER BY random) rn
    FROM details
    WHERE type IN ('a', 'b', 'c')
)

SELECT account, address
FROM cte
WHERE rn <= 50;

如果你真的想采用联合方法，那么下面的语法可能会起作用，每个限制子查询都在一个单独的闭包中：

SELECT account, address
FROM
(SELECT account, address
 FROM details
 WHERE type = 'a'
 ORDER BY random)
UNION ALL
(SELECT account, address
 FROM details
 WHERE type = 'b'
 ORDER BY random)
UNION ALL
(SELECT account, address
 FROM details
 WHERE type = 'c'
 ORDER BY random)

【讨论】：