mongodb group()统计（十）

分组统计：group()           注：不支持分片集群，无法分布式运算

简单聚合：aggregate()       注：支持集群分片（2.2版本增加）

强大统计：mapReduce()       注：支持集群分片（2.4版本增加）

1.计算每个栏目下的商品count()操作

sql写法：select cat_id,count(*) total  from goods group by cat_id

mongodb写法：

db.goods.group({
    key:{cat_id:1},
    cond:{},
    reduce:function(curr,result){
       result.total++;
},
    initial:{total:0}
})

统计价格数大于50块的商品

sql写法：select cat_id,count(*) total from goods where shop_price > 50 group by cat_id

mongodb写法： 

db.goods.group({
    key:{cat_id:1},
    cond:{shop_price:{$gt:50}},
    reduce:function(curr,result){
      result.total++;
},
    initial:{total:0}
})

2.计算每个栏目下的商品库存量sum()操作

sql语法：select cat_id,sum(goods_number) total from goods group by cat_id

mongodb写法：

db.goods.group({
  key:{cat_id:1},
  cond:{},
  reduce:function(curr,result){
       result.goods_number += curr.goods_number;
},
  initial:{goods_number:0}
})

3.计算每个栏目下最贵的商品价格

sql语法：select cat_id,max(shop_price) total from goods group by cat_id

mongodb写法：

db.goods.group({
  key:{cat_id:1},
  cond:{},
  reduce:function(curr,result){
     if(result.max < curr.shop_price){
        result.max = curr.shop_price
}
},
  initial:{max:0}
})

4.查询每个栏目下商品的平均价格

sql写法：select cat_id,avg(shop_price) total from goods group by cat_id

mongodb写法：

db.goods.group({
  key:{cat_id:1},
  cond:{},
  reduce:function(curr,result){
     result.cnt++;
     result.sum += curr.shop_price;
},
  initial:{sum:0,cnt:0},
  finalize:function(result){
     result.avg = result.sum/result.cnt;
}
})