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法律研究如何在 MongoDB 中对多个字段执行独有性查询？
你可以使用 $group 运算符和聚合框架对多个字段执行独有性查询。我们首先使用文档创建一个集合 -
> db.distinctOnMultipleFieldsDemo.insertOne(
... {
... "StudentFirstName" : "Chris",
... "StudentAge" : 21,
... "StudentCountryName": "US"
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd2e518b64f4b851c3a13d7")
}
> db.distinctOnMultipleFieldsDemo.insertOne(
... {
... "StudentFirstName" : "Robert",
... "StudentAge" : 21,
... "StudentCountryName": "AUS"
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd2e518b64f4b851c3a13d8")
}
> db.distinctOnMultipleFieldsDemo.insertOne(
... {
... "StudentFirstName" : "Chris",
... "StudentAge" : 22,
... "StudentCountryName": "UK"
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd2e518b64f4b851c3a13d9")
}
> db.distinctOnMultipleFieldsDemo.insertOne(
... {
... "StudentFirstName" : "David",
... "StudentAge" : 24,
... "StudentCountryName": "AUS"
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd2e518b64f4b851c3a13da")
}以下查询使用 find() 方法从集合中显示所有文档 -
> db.distinctOnMultipleFieldsDemo.find().pretty();
这将产生以下输出 -
{
"_id" : ObjectId("5cd2e518b64f4b851c3a13d7"),
"StudentFirstName" : "Chris",
"StudentAge" : 21,
"StudentCountryName" : "US"
}
{
"_id" : ObjectId("5cd2e518b64f4b851c3a13d8"),
"StudentFirstName" : "Robert",
"StudentAge" : 21,
"StudentCountryName" : "AUS"
}
{
"_id" : ObjectId("5cd2e518b64f4b851c3a13d9"),
"StudentFirstName" : "Chris",
"StudentAge" : 22,
"StudentCountryName" : "UK"
}
{
"_id" : ObjectId("5cd2e518b64f4b851c3a13da"),
"StudentFirstName" : "David",
"StudentAge" : 24,
"StudentCountryName" : "AUS"
}以下查询在 MongoDB 中对多个字段执行独有性查询 -
> db.distinctOnMultipleFieldsDemo.aggregate([
... {
... $group:
... {
... _id:0,
... StudentFirstName: {$addToSet: '$StudentFirstName'},
... StudentAge: {$addToSet: '$StudentAge'},
... StudentCountryName: {$addToSet: '$StudentCountryName'},
... }}
... ]);这将产生以下输出 -
{ "_id" : 0, "StudentFirstName" : [ "David", "Robert", "Chris" ], "StudentAge" : [ 24, 22, 21 ], "StudentCountryName" : [ "UK", "AUS", "US" ] }
更新于：2019 年 7 月 30 日
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