第23章：MongoDB-聚合操作--聚合命令

①count()

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);

②distinct()

范例:求某个字段不同的值

db.students.distinct("name");

db.runCommand({"distinct":"students","key":"name"});

③group()

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 ++;

}

}});

 

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