$setDifference (aggregation)

Definition定义

$setDifference

Takes two sets and returns an array containing the elements that only exist in the first set; i.e. performs a relative complement of the second set relative to the first.获取两个集合并返回一个数组，该数组包含仅存在于第一个集合中的元素；即执行第二集合相对于第一集合的相对补码。

$setDifference has the following syntax:具有以下语法：

{ $setDifference: [ <expression1>, <expression2> ] }

The arguments can be any valid expression as long as they each resolve to an array. For more information on expressions, see Expressions.参数可以是任何有效的表达式，只要它们各自解析为一个数组即可。有关表达式的详细信息，请参阅表达式。

Behavior行为

$setDifference performs set operation on arrays, treating arrays as sets. 对数组执行set操作，将数组视为集合。If an array contains duplicate entries, $setDifference ignores the duplicate entries. 如果数组包含重复的条目，$setDifference将忽略重复的条目。$setDifference ignores the order of the elements.忽略元素的顺序。

$setDifference filters out duplicates in its result to output an array that contain only unique entries. 筛选掉结果中的重复项，以输出仅包含唯一项的数组。The order of the elements in the output array is unspecified.输出数组中元素的顺序未指定。

If a set contains a nested array element, $setDifference does not descend into the nested array but evaluates the array at top-level.如果集合包含嵌套数组元素，则$setDifference不会下降到嵌套数组中，而是在顶级计算数组。

{ $setDifference: [ [ "a", "b", "a" ], [ "b", "a" ] ] }

[ ]

{ $setDifference: [ [ "a", "b" ], [ [ "a", "b" ] ] ] }

[ "a", "b" ]

Example实例

Consider an flowers collection with the following documents:考虑一个包含以下文件的flowers集合：

db.flowers.insertMany( [
   { "_id" : 1, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid" ] },
   { "_id" : 2, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "orchid", "rose", "orchid" ] },
   { "_id" : 3, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid", "jasmine" ] },
   { "_id" : 4, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "jasmine", "rose" ] },
   { "_id" : 5, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ ] },
   { "_id" : 6, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose" ], [ "orchid" ] ] },
   { "_id" : 7, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose", "orchid" ] ] },
   { "_id" : 8, "flowerFieldA" : [ ], "flowerFieldB" : [ ] },
   { "_id" : 9, "flowerFieldA" : [ ], "flowerFieldB" : [ "rose" ] }
] )

The following operation uses the $setDifference operator to return an array of elements found in the flowerFieldB array but not in the flowerFieldA array:以下操作使用$setDifference运算符返回在flowerFieldB数组中找到的元素数组，但不在flowerFieldA数组中找到：

db.flowers.aggregate(
   [
     { $project: { flowerFieldA: 1, flowerFieldB: 1, inBOnly: { $setDifference: [ "$flowerFieldB", "$flowerFieldA" ] }, _id: 0 } }
   ]
)

The operation returns the following results:该操作返回以下结果：

{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid" ], "inBOnly" : [ ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "orchid", "rose", "orchid" ], "inBOnly" : [ ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid", "jasmine" ], "inBOnly" : [ "jasmine" ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "jasmine", "rose" ], "inBOnly" : [ "jasmine" ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ ], "inBOnly" : [ ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose" ], [ "orchid" ] ], "inBOnly" : [ [ "rose" ], [ "orchid" ] ] }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose", "orchid" ] ], "inBOnly" : [ [ "rose", "orchid" ] ] }
{ "flowerFieldA" : [ ], "flowerFieldB" : [ ], "inBOnly" : [ ] }
{ "flowerFieldA" : [ ], "flowerFieldB" : [ "rose" ], "inBOnly" : [ "rose" ] }