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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162
｜
第
7
章
入一个标有
"founded_year"
的文档中呢？这样做的原因是，如果不标注分组的值，那么
就不清楚是按照公司成立的年份进行分组的。为了避免混淆，最佳实践是显式地标注分组
的值。
在某些情况下可能需要使用另一种方法，其中
_id
的值是由多个字段组成的文档。本例实
际上是根据成立年份和类别代码对文档进行分组的：
db.companies.aggregate([
{ $match: { founded_year: { $gte: 2010 } } },
{ $group: {
_id: { founded_year: "$founded_year", category_code: "$category_code" },
companies: { $push: "$name" }
} },
{ $sort: { "_id.founded_year": 1 } }
]).pretty()
在分组阶段使用具有多个字段的文档作为
_id
值是完全没问题的。在某些情况下，以下这
样的做法可能是必需的：
db.companies.aggregate([
{ $group: {
_id: { ipo_year: "$ipo.pub_year" },
companies: { $push: "$name" }
} },
{ $sort: { "_id.ipo_year": 1 } }
]).pretty()
在这种情况下，我们根据公司
IPO
的年份对文档进行分组，而这个年份实际上是内嵌文档 ...
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