MongoDB查询可清楚地计算数组的大小？
使用DISTINCT表示不同的元素，然后使用length获得数组的大小-
db.yourCollectionName.distinct('yourFieldName').length;首先让我们创建一个包含文档的集合-
> db.countOrSizeDemo.insertOne({"StudentFirstName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304f5b64f4b851c3a13dc")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304fab64f4b851c3a13dd")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304fcb64f4b851c3a13de")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"Carol"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd30500b64f4b851c3a13df")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"Sam"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd30505b64f4b851c3a13e0")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd3050ab64f4b851c3a13e1")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.countOrSizeDemo.find();
这将产生以下输出-
{ "_id" : ObjectId("5cd304f5b64f4b851c3a13dc"), "StudentFirstName" : "John" }
{ "_id" : ObjectId("5cd304fab64f4b851c3a13dd"), "StudentFirstName" : "David" }
{ "_id" : ObjectId("5cd304fcb64f4b851c3a13de"), "StudentFirstName" : "David" }
{ "_id" : ObjectId("5cd30500b64f4b851c3a13df"), "StudentFirstName" : "Carol" }
{ "_id" : ObjectId("5cd30505b64f4b851c3a13e0"), "StudentFirstName" : "Sam" }
{ "_id" : ObjectId("5cd3050ab64f4b851c3a13e1"), "StudentFirstName" : "John" }以下是获取查询数组大小的查询-
> db.countOrSizeDemo.distinct('StudentFirstName').length;这将产生以下输出-
4