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Legal Studies如何使用 MongoDB 统计数组中的项？
要统计数组中的项，请使用 length。让我们用文档创建一个集合 -
> db.demo440.insertOne(
... {
... "Name":"Chris",
... "ListOfFriends":["John","Sam","Mike"]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e78c63cbbc41e36cc3caeb5")
}
>
> db.demo440.insertOne(
... {
... "Name":"David",
... "ListOfFriends":["Mike","Bob","Carol"]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e78c63cbbc41e36cc3caeb6")
}借助 find() 方法从集合中显示所有文档 -
> db.demo440.find();
这将产生以下输出 -
{ "_id" : ObjectId("5e78c63cbbc41e36cc3caeb5"), "Name" : "Chris", "ListOfFriends" : [ "John", "Sam", "Mike" ] }
{ "_id" : ObjectId("5e78c63cbbc41e36cc3caeb6"), "Name" : "David", "ListOfFriends" : [ "Mike", "Bob", "Carol" ] }以下是统计数组中项的查询 -
>db.demo440.findOne({'Name':'David'}).ListOfFriends.length;这将产生以下输出 -
3
更新于： 06-Apr-2020
237 次查看
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