范例：统计students表中的数据量
db.students.count();
范例：模糊查询
db.students.count("name":/张/i);
范例:求某个字段不同的值
db.students.distinct("name");
db.runCommand({"distinct":"students","key":"name"});
key: 用于指定要分组的键
initial: 对于分组统计的字段设置键名和初始值
reduce: 循环每个分组中的每个文档,一组循环完了会继续下一组
condition：用于分组前筛选掉不满足条件的文档
finalize: 用于对分组后的结果进一步处理,每组都会调用finalize
db.articles.group({
"key": {"author": true},
"initial": {"sum": 0},
"reduce": function(doc, prev) {
if(doc.like > 10) {
prev.sum += 1;
}
}
})
[
{
"author" : "zhangsan",
"sum" : 2
},
{
"author" : "lisi",
"sum" : 2
},
{
"author" : "mengday",
"sum" : 0
}
]
db.articles.group({
"key": {"author": true},
"initial": {"sum": 0},
"reduce": function(doc, prev) {
if(doc.like > 10) {
prev.sum += 1;
}
},
"condition":{"author": {"$ne": "mengday"}}
})
[
{
"author" : "zhangsan",
"sum" : 2
},
{
"author" : "lisi",
"sum" : 2
}
]
db.articles.group({
"key": {"author": true},
"initial": {"sum": 0},
"reduce": function(doc, prev) {
if(doc.like > 10) {
prev.sum += 1;
}
},
"condition":{"author": {"$ne": "mengday"}},
"finalize": function(doc){
if(doc.author == "zhangsan"){
delete doc
}
}
})
[
{
"author" : "zhangsan",
"sum" : 3
},
{
"author" : "lisi",
"sum" : 2
}
]
范例：查询所有年龄大于等于19岁的学生信息，并且按照年龄分组
db.runCommand({"group":{
"ns":"students",//集合
"key":{"age":true},//按照age分组
"initial":{"count":0},//默认数量为0
"condition":{"age":{"$gt"：19}},//条件
"$reduce":function(doc,prev){//完成后的操作
prev.count ++;
}
}});
来源：https://www.cnblogs.com/Lucky-stars/p/10555313.html