$min (aggregation)¶

$min¶

The $min operator returns the lowest non-null value of a field in the documents for a $group operation.

Changed in version 2.4: 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

The users collection contains the following documents:

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{ "_id" : "abc001", "age" : 25 }
{ "_id" : "abe001", "age" : 35 }
{ "_id" : "efg001", "age" : 20 }
{ "_id" : "xyz001", "age" : 15 }

To find the minimum value of the age field from all the documents, use the $min operator:

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db.users.aggregate( [ { $group: { _id:0, minAge: { $min: "$age"} } } ] )

The operation returns the value of the age field in the minAge field:

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{ "result" : [ { "_id" : 0, "minAge" : 15 } ], "ok" : 1 }

To find the minimum value of the age field for only those documents with _id starting with the letter a, use the $min operator after a $match operation:

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db.users.aggregate( [ { $match: { _id: /^a/ } },
                      { $group: { _id: 0, minAge: { $min: "$age"} } }
                    ] )

The operation returns the minimum value of the age field for the two documents with _id starting with the letter a:

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{ "result" : [ { "_id" : 0, "minAge" : 25 } ], "ok" : 1 }

Example

The users collection contains the following documents where some of the documents are either missing the age field or the age field contains null:

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{ "_id" : "abc001", "age" : 25 }
{ "_id" : "abe001", "age" : 35 }
{ "_id" : "efg001", "age" : 20 }
{ "_id" : "xyz001", "age" : 15 }
{ "_id" : "xxx001" }
{ "_id" : "zzz001", "age" : null }

The following operation finds the minimum value of the age field in all the documents:
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```
db.users.aggregate( [ { $group: { _id:0, minAge: { $min: "$age"} } } ] )
```
Because only some documents for the $min operation are missing the age field or have age field equal to null, $min only considers the non-null and the non-missing values and the operation returns the following document:
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```
{ "result" : [ { "_id" : 0, "minAge" : 15 } ], "ok" : 1 }
```

The following operation finds the minimum value of the age field for only those documents where the _id equals "xxx001" or "zzz001":

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db.users.aggregate( [ { $match: { _id: {$in: [ "xxx001", "zzz001" ] } } },
                      { $group: { _id: 0, minAge: { $min: "$age"} } }
                    ] )

The $min operation returns null for the minimum age since all documents for the $min operation have null value for the field age or are missing the field:

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{ "result" : [ { "_id" : 0, "minAge" : null } ], "ok" : 1 }

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