从MongoDB获取随机记录？
要从MongoDB获取随机记录，可以使用聚合函数。语法如下：
db.yourCollectionName.aggregate([{$sample:{size:1}}]);为了理解上述语法，让我们创建包含一些文档的集合。创建集合的查询如下：
>db.employeeInformation.insert({"EmployeeId":1,"EmployeeName":"Maxwell","EmployeeAge":26});
WriteResult({ "nInserted" : 1 })
>db.employeeInformation.insert({"EmployeeId":2,"EmployeeName":"David","EmployeeAge":25});
WriteResult({ "nInserted" : 1 })
>db.employeeInformation.insert({"EmployeeId":3,"EmployeeName":"Carol","EmployeeAge":24});
WriteResult({ "nInserted" : 1 })
>db.employeeInformation.insert({"EmployeeId":4,"EmployeeName":"Bob","EmployeeAge":28});
WriteResult({ "nInserted" : 1 })
>db.employeeInformation.insert({"EmployeeId":5,"EmployeeName":"Sam","EmployeeAge":27);
WriteResult({ "nInserted" : 1 })现在，您可以借助find()method显示集合中的所有文档。查询如下：
> db.employeeInformation.find().pretty();
以下是输出：
{
"_id" : ObjectId("5c6d3079734e98fc0a434ae5"),
"EmployeeId" : 1,
"EmployeeName" : "Maxwell",
"EmployeeAge" : 26
}
{
"_id" : ObjectId("5c6d308c734e98fc0a434ae6"),
"EmployeeId" : 2,
"EmployeeName" : "David",
"EmployeeAge" : 25
}
{
"_id" : ObjectId("5c6d309d734e98fc0a434ae7"),
"EmployeeId" : 3,
"EmployeeName" : "Carol",
"EmployeeAge" : 24
}
{
"_id" : ObjectId("5c6d30ab734e98fc0a434ae8"),
"EmployeeId" : 4,
"EmployeeName" : "Bob",
"EmployeeAge" : 28
}
{
"_id" : ObjectId("5c6d30bb734e98fc0a434ae9"),
"EmployeeId" : 5,
"EmployeeName" : "Sam",
"EmployeeAge" : 27
}这是获取随机记录的查询：
> db.employeeInformation.aggregate([{$sample:{size:1}}]).pretty();以下是输出：
{
"_id" : ObjectId("5c6d30bb734e98fc0a434ae9"),
"EmployeeId" : 5,
"EmployeeName" : "Sam",
"EmployeeAge" : 27
}让我们使用相同的查询获得另一个随机记录：
> db.employeeInformation.aggregate([{$sample:{size:1}}]).pretty();以下是输出：
{
"_id" : ObjectId("5c6d308c734e98fc0a434ae6"),
"EmployeeId" : 2,
"EmployeeName" : "David",
"EmployeeAge" : 25
}