MongoDB聚合运算符：$setDifference

$setDifference聚合运算符比较两个数组，返回之前第一个数组中存在的元素，相当于返回第二个数组对于第一个数组的补集。

语法

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

参数可以是任何能够解析为数组的表达式。

使用

• $setDifference 对数组执行集合操作，将数组视为集合。如果数组包含重复元素，$setDifference 会忽略重复元素。 $setDifference 忽略元素的顺序。
• $setDifference 会过滤掉结果中的重复元素，输出仅包含唯一元素的数组，输出数组中元素的顺序未指定。
• 如果集合包含嵌套数组元素，则 $setDifference 不会下降到嵌套数组，而只处理顶层数组元素。

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

举例

使用下面的脚本创建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" ] }
] )

下面的聚合使用 $setDifference 运算符返回在flowerFieldB 数组中找到但在flowerFieldA 数组中未找到的元素的数组：

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

操作返回下面的结果：

{ "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" ] }