$min (aggregation)¶

$min¶

Returns the lowest value that results from applying an expression to each document in a group of documents that share the same group by key.

$min is an accumulator operator available only in the $group stage.

$min has the following syntax:

{ $min: <expression> }

For more information on expressions, see Expressions.

Behavior¶

在 2.4 版更改.

If some, but not all, documents for the $min operation have either a null value for the field or are missing the field, the $min operator only considers the non-null and the non-missing values for the field.

If all documents for the $min operation have null value for the field or are missing the field, the $min operator returns null for the minimum value.

Before 2.4, if any of the documents for the $min operation were missing the field, the $min operator would not return any value. If any of the documents for the $min had the value null, the $min operator would return a null.

Example¶

Consider a sales collection with the following documents:

{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }

Grouping the documents by the item field, the following operation uses the $min accumulator to compute the minimum amount and minimum quantity for each grouping.

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: "$item",
           minQuantity: { $min: "$quantity" }
         }
     }
   ]
)

The operation returns the following results:

{ "_id" : "xyz", "minQuantity" : 5 }
{ "_id" : "jkl", "minQuantity" : 1 }
{ "_id" : "abc", "minQuantity" : 2 }

← $max (aggregation) $push (aggregation) →