MongoDB是否相当于SELECT字段AS“另一个名称”？
在MySQL中，我们为列指定别名。同样，您可以在MongoDB中为字段名称提供别名。MongoDB的等效语法如下
db.yourCollectionName.aggregate(
[
{ "$project": {
"_id": 0,
"anyAliasName": "$yourFieldName"
}}
]);首先让我们创建一个包含文档的集合
> db.selectFieldAsAnotherNameDemo.insertOne({"Name":"Larry"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9d448827b86948e204ca91")
}
> db.selectFieldAsAnotherNameDemo.insertOne({"Name":"Robert"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9d449027b86948e204ca92")
}
> db.selectFieldAsAnotherNameDemo.insertOne({"Name":"Sam"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9d449527b86948e204ca93")
}
> db.selectFieldAsAnotherNameDemo.insertOne({"Name":"Mike"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9d449927b86948e204ca94")
}以下是在find()方法的帮助下显示集合中所有文档的查询
> db.selectFieldAsAnotherNameDemo.find().pretty();
这将产生以下输出
{ "_id" : ObjectId("5c9d448827b86948e204ca91"), "Name" : "Larry" }
{ "_id" : ObjectId("5c9d449027b86948e204ca92"), "Name" : "Robert" }
{ "_id" : ObjectId("5c9d449527b86948e204ca93"), "Name" : "Sam" }
{ "_id" : ObjectId("5c9d449927b86948e204ca94"), "Name" : "Mike" }以下是对与SELECT字段AS`anothername`相对应的MongoDB的查询
> db.selectFieldAsAnotherNameDemo.aggregate(
... [
... { "$project": {
... "_id": 0,
... "StudentName": "$Name"
... }}
... ]);这将产生以下输出
{ "StudentName" : "Larry" }
{ "StudentName" : "Robert" }
{ "StudentName" : "Sam" }
{ "StudentName" : "Mike" }