November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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140
｜
第
7
章
再次回顾一下此管道。这个管道有
5
个阶段。首先，过滤
companies
集合，只寻找
"founded_year"
为
2004
的文档。然后，根据名称升序排列，跳过前
10
个匹配项，并将最
终结果限制为
5
个
。最后，将这
5
个文档传递到投射阶段，在这个阶段会重新调整文档形
状，使输出的文档只包含公司名称。
这里介绍了如何使用你已经熟悉的操作所形成的阶段来构建管道。聚合框架提供了这些操
作，它们对于后文讨论的阶段所要完成的分析类型是必要的。接下来，本章会深入讲解聚
合框架所提供的其他操作。
7.3
表达式
随着对聚合框架的深入讨论，在构建聚合管道时，了解可以使用的不同类型的表达式是很
重要的。聚合框架支持许多表达式类型。
•
布尔
表达式允许使用
AND
、
OR
和
NOT
。
•
集合
表达式允许将数组作为集合来处理。特别地，可以取两个或多个集合的交集或并集，
也可以取两个集合的差值并执行一些其他的集合运算。
•
比较
表达式能够表达许多不同类型的范围过滤器。
•
算术
表达式能够计算上限（
ceil
ing
）、下限（
floor
）、自然对数和对数，以及执行简单的
算术运算，比如乘法、除法、加法和减法。甚至可以执行更复杂的运算，比如计算值的
平方根。
•
字符串
表达式允许连接、查找子字符串，以及执行与大小写和文本搜索相关的操作。
•
数组
表达式为操作数组提供了强大的功能，包括过滤数组元素、对数组进行分割或从特
定数组中获取某一个范围的值。
•
变量
表达式在本书中不会深入研究，这类表达式允许处理文字、解析日期值及条件表
达式。
•
累加器
提供了计算总和、描述性统计和许多其他类型值的能力。 ...
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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.