聚合运算之group
语法：
db.collection.group(
{
key:{key1:1,key2:1},
cond:{},
reduce: function(curr,result) {
},
initial:{},
finalize:function() {
}
}
)
key: 分组字段
cond:查询条件
reduce:聚合函数
initial:初始化
finalize:统计一组后的回调函数
#查询每个栏目下的商品数量
db.goods.group(
{
key:{cat_id:1},
cond:{},
reduce:function(curr,result) {
result.cnt += 1;
},
initial:{cnt:0}
}
)
#查询每个栏目下价格高于50元的商品数量
db.goods.group(
{
key:{cat_id:1},
cond:{shop_price:{$gt:50}},
reduce:function(curr,result) {
result.cnt += 1;
},
initial:{cnt:0}
}
)
#每个栏目下的商品库存量 sum()操作
db.goods.group(
{
key:{cat_id:1},
cond:{},
reduce:function(curr,result) {
result.num += curr.goods_number;
},
initial:{num:0}
}
)
#查询每个栏目最贵的商品价格, max()操作
db.goods.group(
{
key:{cat_id:1},
cond:{},
reduce:function(curr , result) {
if(curr.shop_price > result.max) {
result.max = curr.shop_price;
}
},
initial:{max:0}
}
)
#查询每个栏目下商品的平均价格
db.goods.group(
{
key:{cat_id:1},
cond:{},
reduce:function(curr , result) {
result.cnt += 1;
result.sum += curr.shop_price;
},
initial:{sum:0,cnt:0},
finalize:function(result) {
result.avg = result.sum/result.cnt;
}
}
)
使用aggregate聚集框架简单聚合
#查询每个栏目下的商品数量
db.collection.aggregate
(
[
{$group:{_id:"$cat_id",total:{$sum:1}}}
]
);
#查询goods下有多少条商品,select count(*) from goods
db.collection.aggregate
(
[
{$group:{_id:null,total:{$sum:1}}}
]
)
#查询每个栏目下 价格大于50元的商品个数
db.collection.aggregate
(
[
{$match:{shop_price:{$gt:50}}},
{$group:{_id:"$cat_id",total:{$sum:1}}}
]
)
#查询每个栏目下 价格大于50元的商品个数
#并筛选出"满足条件的商品个数" 大于等于3的栏目
db.collection.aggregate
(
[
{$match:{shop_price:{$gt:50}}},
{$group:{_id:"$cat_id",total:{$sum:1}}},
{$match:{total:{$gte:3}}}
]
)
#查询每个栏目下的库存量
db.collection.aggregate
(
[
{$group:{_id:"$cat_id" , total:{$sum:"$goods_number"}}},
]
)
#查询每个栏目下的库存量,并按库存量排序
db.collection.aggregate
(
[
{$group:{_id:"$cat_id" , total:{$sum:"$goods_number"}}},
{$sort:{total:1}}
]
)
#查询每个栏目下的库存量,并按库存量排序
db.collection.aggregate
(
[
{$group:{_id:"$cat_id" , total:{$sum:"$goods_number"}}},
{$sort:{total:1}},
{$limit:3}
]
)
#查询每个栏目的商品平均价格,并按平均价格由高到低排序
db.collection.aggregate
(
[
{$group:{_id:"$cat_id" , avg:{$avg:"$shop_price"}}},
{$sort:{avg:-1}}
]
)