开心档-软件开发入门之MongoDB 聚合

>db.COLLECTION_NAME.aggregate(AGGREGATE_OPERATION)

{
   _id: ObjectId(7df78ad8902c)
   title: 'MongoDB Overview', 
   description: 'MongoDB is no sql database',
   by_user: 'kxdang.com',
   url: 'http://www.kxdang.com/topic/',
   tags: ['mongodb', 'database', 'NoSQL'],
   likes: 100
},
{
   _id: ObjectId(7df78ad8902d)
   title: 'NoSQL Overview', 
   description: 'No sql database is very fast',
   by_user: 'kxdang.com',
   url: 'http://www.kxdang.com/topic/',
   tags: ['mongodb', 'database', 'NoSQL'],
   likes: 10
},
{
   _id: ObjectId(7df78ad8902e)
   title: 'Neo4j Overview', 
   description: 'Neo4j is no sql database',
   by_user: 'Neo4j',
   url: 'http://www.neo4j.com',
   tags: ['neo4j', 'database', 'NoSQL'],
   likes: 750
},

> db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$sum : 1}}}])
{
   "result" : [
      {
         "_id" : "kxdang.com",
         "num_tutorial" : 2
      },
      {
         "_id" : "Neo4j",
         "num_tutorial" : 1
      }
   ],
   "ok" : 1
}
>

 select by_user, count(*) from mycol group by by_user

db.article.aggregate(
    { $project : {
        title : 1 ,
        author : 1 ,
    }}
 );

db.article.aggregate(
    { $project : {
        _id : 0 ,
        title : 1 ,
        author : 1
    }});

db.articles.aggregate( [
                        { $match : { score : { $gt : 70, $lte : 90 } } },
                        { $group: { _id: null, count: { $sum: 1 } } }
                       ] );

db.article.aggregate(
    { $skip : 5 });