MongoDB 聚合

例子：
    找出发表文章量前五的作者


sql：
    select author,count(1) 
    from articles
    group by author
    order by count desc
    limit 5;

mongo:
    db.articles.aggeregate(
        {'$project':{'author':1}},
        {'$group':{'_id':'$author','count':{'$sum':1}}},
        {'$sort':{'count':-1}},
        {'$limit':5}
    )

实际例子：

db.wait_bind.aggregate(
    {'$project':{'state':1}},
    {'$group':{'_id':'$state','count':{'$sum':1}}},
    {'$sort':{'count':-1}}
)