1.分组
> db.student.aggregate([{$group:{_id:'$class',total:{$sum:1}}}]);
{ "_id" : 121, "total" : 3 }
{ "_id" : 122, "total" : 3 }
{ "_id" : 123, "total" : 3 }
2.分组后过滤
查出班级大于22的是哪几个班 分别是多少
db.student.aggregate([{$match:{class:{$gt:122}}},{$group:{_id:'$class',total:{$sum:1}}}]);
{ "_id" : 125, "total" : 1 }
{ "_id" : 123, "total" : 4 }
> db.student.aggregate([{$group:{_id:'$class',total:{$sum:1}}}]);
{ "_id" : 121, "total" : 3 }
{ "_id" : 122, "total" : 3 }
{ "_id" : 123, "total" : 3 }
2.分组后过滤
查出班级大于22的是哪几个班 分别是多少
db.student.aggregate([{$match:{class:{$gt:122}}},{$group:{_id:'$class',total:{$sum:1}}}]);
{ "_id" : 125, "total" : 1 }
{ "_id" : 123, "total" : 4 }