nosql - MongoDB 官方文档中的 aggregate 例子当中的 $sum: 1 ， 这里的 1 起什么作用？

Question

db.restaurants.aggregate( 
[ 
{ $match: { "borough": "Queens", "cuisine": "Brazilian" } }, 
{ $group: { "_id": "$address.zipcode" , "count": { $sum: 1 } } } 
]
);

Answer 1

http://docs.mongodb.org/manual/reference/operator/aggregation/sum/

Sum by expression value 1 represents 1

Answer 2

According to the conditions of the group, if one condition is met, add 1, that is, count is the number of each zipcode in the group