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Legal StudiesMongoDB 查询如何才能统计数组的大小？
使用 DISTINCT 获取唯一元素，再用 length 获取数组的大小 −
db.yourCollectionName.distinct('yourFieldName').length;我们先用文档创建一个集合 −
> 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")
}以下查询使用 find() 方法显示集合中的所有文档 −
> 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" }以下查询获取查询数组的大小 −
> db.countOrSizeDemo.distinct('StudentFirstName').length;这将产生以下输出 −
4
更新于: 30-Jul-2019
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