MongoDB学习笔记05

count 返回集合中文档数量文档数量

db.foo.count()
db.foo.count({"x":2})

distinct用来找出给定键的所有不同的值，使用时必须指定集合和键

db.runCommand({"distinct":"person","key","age"})
db.person.distinct("age")

group的聚合要先选定分组所依据的键【键要是个对象才能返回所有的分组结果】，而后MongoDB就会将集合依据选定键值的不同分成若干组  condition 满足指定条件的文档

db.runCommand({"group":{ "ns":"stocks", "key":{"day":true}, "initial":{"time":0}, "$reduce":function(doc,prev){   if(d
oc.time>prev.time){     prev.price=doc.price;     prev.time=doc.time; } }}})

db.runCommand({"group":{ "ns":"stocks", "key":"day", "initial":{"time":0}, "$reduce":function(doc,prev){   if(d
oc.time>prev.time){     prev.price=doc.price;     prev.time=doc.time; } }}})

ps：ns 指定要进行分组的集合 key 指定分组依据的键 initial 每一组reduce函数调用的初始时间，会作为初始文档传递给后续过程  $reduce 每个文档都对应一次这个调用

结果：

{
        "retval" : [
                {
                        "day" : "2010/1/1",
                        "time" : "1",
                        "price" : 1
                },
                {
                        "day" : "2010/1/2",
                        "time" : "1",
                        "price" : 1
                },
                {
                        "day" : "2010/1/3",
                        "time" : "1",
                        "price" : 1
                }
        ],
        "count" : NumberLong(4),
        "keys" : NumberLong(3),
        "ok" : 1
}


{
        "retval" : [
                {
                        "time" : "1",
                        "price" : 1
                }
        ],
        "count" : NumberLong(4),
        "keys" : NumberLong(1),
        "ok" : 1
}