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$setDifference（表达式操作符）

定义

$setDifference

取两个集并返回一个包含仅存在于第一个集中的元素的数组；即，对第二个集取得相对于第一个集的相对补集。

$setDifference 通过以下语法实现：

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

参数可以是任何有效的表达式，只要它们都解析为一个数组即可。有关表达式的更多信息，请参阅表达式。

行为

$setDifference performs set operation on arrays, treating arrays as sets. If an array contains duplicate entries, $setDifference ignores the duplicate entries. $setDifference ignores the order of the elements.

$setDifference 过滤掉结果中的重复项，以输出仅包含唯一条目的数组。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.

例子

结果

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

[ ]

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

[ "a", "b" ]

例子

考虑包含以下文档的 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:

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

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" ] }