MongoDB数据库，相当于SELECT列1，列2 FROM tbl [英] MongoDB Database, equivalent for SELECT column1, column2 FROM tbl

问题描述

在我的MongoDB中，我希望得到与之对应的

From my MongoDB I want the equivalent for

  SELECT column1, column2
  FROM tbl

使用此代码，我得到了所有的行"，但也得到了所有的列"

With this code I get all the 'rows' but also all the 'columns'

  DBCollection collection = database.getCollection("names");
  DBCursor cursor = collection.find();

例如，

我想要全部行"，但仅列":ID，名称，年龄

I for example would like all the 'rows' but only the 'columns': id, name, age

我该怎么做?

感谢您的帮助！

推荐答案

db.collection.find({}，{_id:1，名称:1，年龄:1})

db.collection.find({}, {_id: 1, name: 1, age: 1})

要查找(谓词)的第一个参数是您的选择条件，例如

The first argument to find (the predicate) is your selection criteria, e.g.

db.collection.find({age:{$ gte:21}})

db.collection.find({age: {$gte: 21}})

第二个字符限制了您检索的字段，因此对于21岁或21岁以上的所有人来说都是这样:

The second limits the fields you retrieve, so for the names over everyone 21 or older:

db.collection.find({age:{$ gte:21}}，{name:1})

db.collection.find({age: {$gte: 21}}, {name: 1})

除非您专门将其关闭，否则字段选择器始终拉回_id:

The field selector always pulls back _id unless you specifically turn it off:

db.collection.find({}，{_id:0})

db.collection.find({}, {_id: 0})

但是，默认情况下，Mongo不会检查字段是否存在.如果要选择某些字段，并且仅匹配具有这些字段的结果，则将要使用:

However, Mongo will not check for field existence by default. If you want to select certain fields, and match only results that have those fields, you will want to use:

db.collection.find({age:{$ exists:true}})

db.collection.find({ age: { $exists: true } })

MongoDB网站对.find()函数有更详细的描述！

The MongoDB website has a more detailed description of the .find() function!

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