和

如何获得 amount 的总和？请参阅以下聚合查询。

> db.transactions.aggregate( 
     [
         {
             $group : {
                 _id : '$cr_dr',
                 count : {$sum : 1},    //counts the number
                 totalAmount : {$sum : '$amount'}    //sums the amount
             }
         }
     ] 
 );

结果是

{
    "_id" : "C",
    "count" : 3.0,
    "totalAmount" : 120.0
}
{
    "_id" : "D",
    "count" : 5.0,
    "totalAmount" : 410.0
}

另一个版本总结了 amount 和 fee。

> db.transactions.aggregate(
      [
          {
              $group : {
                  _id : '$cr_dr',
                  count : {$sum : 1},
                  totalAmount : {$sum : { $sum : ['$amount', '$fee']}}
              }
          }
      ] 
  );

结果是

{
    "_id" : "C",
    "count" : 3.0,
    "totalAmount" : 128.0
}    
{
    "_id" : "D",
    "count" : 5.0,
    "totalAmount" : 422.0
}