March 2021
Intermediate to advanced
265 pages
6h 50m
Chinese
Content preview from Presto实战
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高级
SQL
特性
｜
165
9.18
聚合函数
在
SQL
中，
聚合函数
操作并计算出一个值或一组值。与标量函数对每个输入值产生单个值
不同，聚合函数对一组输入值产生单个值。
Presto
支持大多数其他数据库系统中常见的通
用聚合函数，如表
9-15
所示。
聚合函数可以在参数后加入一个可选的
ORDER BY
子句。在语义上，这意味着在执行聚合之
前对输入集进行排序。对于大多数聚合函数来说，顺序并不重要。
表9-15：聚合函数
函 数 返回类型 描 述
count(
*
) BIGINT
返回传入数值的个数
count(
x
) BIGINT
返回传入非
NULL
值的个数
sum(
x
)
同输入 返回传入值的总和
min(
x
)
同输入 返回传入值的最小值
max(
x
)
同输入 返回传入值的最大值
avg(
x
) DOUBLE
返回传入值的平均值
9.18.1
映射聚合函数
Presto
支持几个有用的映射相关函数，见表
9-16
。对于其中一些函数，根据所需的结果，
需要使用可选的
ORDER BY
子句。我们用鸢尾花数据集（参见
1.4.7
节）的例子来演示这种
使用方法。
表9-16：映射聚合函数
函 数 返回类型 描 述
histogram(
x
)
map(
K
,
bigint
)
这个函数从传入值
x
中创建一个直方图，返回一个
映射，其中键是
x
，值是
x
出现的次数
map_agg(
key
,
value
)
map(
K
,
V
)
从一组键值中创建映射，重复键对应的值是随机选
取的，可以使用
multimap_agg
来生成单键对多值的
映射
map_union(
x
(
K
,
V
)) map(
K
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.