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MongoDB基础教程系列--第五篇 MongoDB 映射与限制记录

上一篇提到的 find() 的方法,细心的伙伴会发现查询的结果都是显示了集合中全部的字段,实际应用中,显然是不够用的。那么有没有办法指定特定的字段显示出文档呢?答案是肯定的,MongoDB 中用映射实现这种功能。

1、映射

MongoDB 中限制字段的显示,可以利用 0 或 1 来设置字段列表。1 用于显示字段,0 用于隐藏字段。

格式

db.COLLECTION_NAME.find({},{KEY:1})

范例

查询文档时,只显示文档中的 name。首先查询出 user 集合中的所有文档,然后利用映射,返回文档中 name 字段。

> db.user.find(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }>db.user.find({},{"name":1, "_id":0}){ "name" : "liruihuan" }{ "name" : "user1" }{ "name" : "user2" }{ "name" : "user3" }>

如果不加 "_id":0 会返回什么结果呢

> db.user.find({},{"name":1}){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3" }>

我们发现不设置 "_id":1 结果中也会返回 _id 字段,这是因为在执行 find() 方法时,_id 字段是一直显示的。如果不想显示该字段,则可以设置 "_id":0。

2、限制记录

MongoDB 中想要显示或者跳过指定的文档条数,可以利用 limit() 方法和 skip() 方法

2.1、limit() 方法

limit() 方法接受一个数值类型的参数,其值为想要显示的文档数。

格式

db.COLLECTION_NAME.find().limit(NUMBER)

范例

查询文档时,只显示两条文档。首先查询出 user 集合中的所有文档,然后利用 limit() 方法,显示两条记录。

> db.user.find(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }>db.user.find().limit(2){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }>

如果不给 limit() 指定参数呢,我们会发现,返回了全部文档。

> db.user.find().limit(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }

2.2、skip() 方法

skip() 方法接受一个数值类型的参数,其值为想要跳过的文档数。

格式

db.COLLECTION_NAME.find().limit(NUMBER).skip(NUMBER)

范例

查询文档时,只显示第二条文档。首先查询出 user 集合中的所有文档,然后利用 limit(1) 方法,显示一条文档,利用 skip(1) 方法跳过第一条文档。

> db.user.find(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }>db.user.find().limit(1).skip(1){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }>

skip() 方法的默认值是 0 。

> db.user.find(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }> db.user.find().skip(){ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754b"), "name" : "liruihuan", "age" : 18,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754d"), "name" : "user1", "age" : 19,"sex":"man" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754e"), "name" : "user2", "age" : 20,"sex":"woman" }{ "_id" : ObjectId("58e1d2f0bb1bbc3245fa754f"), "name" : "user3", "age" : 19,"sex":"woman" }>

 

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MongoDB基础教程系列--第五篇 MongoDB 映射与限制记录