如何计算 MongoDB 中每个字段/键的不同值的数量？
您可以为此使用distinct命令。为了理解这个概念，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.distinctCountValuesDemo.insertOne({"StudentFirstName":"John","StudentFavouriteSubject":["C","C++","Java","MySQL","C","C++"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a39f193b406bd3df60e07")
}
> db.distinctCountValuesDemo.insertOne({"StudentFirstName":"Larry","StudentFavouriteSubject":["MongoDB","SQL Server"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a3a1193b406bd3df60e08")
}在find()方法的帮助下显示集合中的所有文档。查询如下-
> db.distinctCountValuesDemo.find().pretty();
以下是输出-
{
"_id" : ObjectId("5c8a39f193b406bd3df60e07"),
"StudentFirstName" : "John",
"StudentFavouriteSubject" : [
"C",
"C++",
"Java",
"MySQL",
"C",
"C++"
]
}
{
"_id" : ObjectId("5c8a3a1193b406bd3df60e08"),
"StudentFirstName" : "Larry",
"StudentFavouriteSubject" : [
"MongoDB",
"SQL Server"
]
}这是查找每个字段/键的多个不同值的查询-
> db.distinctCountValuesDemo.distinct('StudentFavouriteSubject');以下是输出-
[ "C", "C++", "Java", "MySQL", "MongoDB", "SQL Server" ]
这是查找数组中不同值长度的查询-
> db.distinctCountValuesDemo.distinct('StudentFavouriteSubject').length;以下是输出-
6