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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45
第 4 章
查询
本章会详细介绍查询，主要涵盖以下几个方面：
•
使用
$
条件进行范围查询、数据集包含查询、不等式查询，以及其他一些查询；
•
查询会返回一个数据库游标，其只会在需要时才惰性地进行批量返回；
•
有很多可以针对游标执行的元操作，包括跳过一定数量的结果、限定返回结果的数量，
以及对结果进行排序。
4.1
find
简介
我们在
MongoDB
中使用
find
方法来进行查询。查询就是返回集合中文档的一个子集，子
集的范围从
0
个文档到整个集合。要返回哪些文档由
find
的第一个参数决定，该参数是一
个用于指定查询条件的文档。
空的查询文档（
{}
）会匹配集合中的所有内容。如果
find
没有给定查询文档，则默认为
{}
。
例如：
> db.c.find()
将匹配集合
c
中的所有文档（并批量返回）。
当开始向查询文档中添加键
–
值对时
，就意味着限定了查询条件。这对于大多数类型来说
是一种简单明了的方式：数值匹配数值，布尔类型匹配布尔类型，字符串匹配字符串。查
询简单类型只要指定要查找的值就可以了。例如，要查找
"age"
值为
27
的所有文档，可以
将该键
–
值对添加到查询文档中：
> db.users.find({"age" : 27})
46
｜
第
4
章
如果有一个要匹配的字符串，比如键
"username"
和它的值
"joe"
，则可以使用该键 ...
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