// 準備測試數據db.user.drop();for(var i=10; i< 100; i++) {  db.user.insert({    name:"user" + i,     age : Math.floor(Math.random()*10)+ 20,     sex : Math.floor(Math.random()*3)%2 ==0 ? 'M' : 'F',    chinese : Math.floor(Math.random()*50)+50,    math : Math.floor(Math.random()*50)+50,    english : Math.floor(Math.random()*50)+50,    class : "C" + i%5  })}// group函數// 按照class進行分組，顯示每個class中的用戶姓名和性別db.user.group({  key: {"class": true},  initial: {"person": []},  reduce: function(cur, prev) {    prev.person.push({name: cur.name, sex: cur.sex, age: cur.age});  }});// 對age>25的用戶，按照class進行分組，顯示每個class中的用戶姓名和性別，并統計每組的人數db.user.group({  key: {"class": true},  initial: {"person": []},  reduce: function(doc, out){    out.person.push({name: doc.name, sex: doc.sex, age: doc.age});  },  finalize: function(out){    out.count = out.person.length;  },  condition: {"age": {$gt: 25}}})// 分組計算每個class中，chinese最大值和最小值db.user.group({  key: {"class": true},  initial: {"chinese_min": 0, "chinese_max":0 },  reduce: function(doc, out){    out.chinese_min = doc.chinese;    out.chinese_min = doc.chinese;    out.chinese_min = Math.min(out.chinese_min, doc.chinese);    out.chinese_max = Math.max(out.chinese_max, doc.chinese)  },})// 利用分組，計算每個總成績和成績平均值db.user.group({  key: {"_id" : true},  initial: {name:"", total: 0, avg: 0},  reduce: function(doc, out){    out.name = doc.name;    out.total = doc.chinese + doc.math + doc.english;    out.avg = Math.floor(out.total / 3);  }})