Database Manual / Reference / Query Language / Expressions

$setDifference (aggregation)（聚合）

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.取两个集合，并返回一个数组，其中包含仅存在于第一个集合中的元素；即，执行第二组相对于第一组的相对补码。

$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. 参数可以是任何有效的表达式，只要它们各自解析为数组即可。For more information on expressions, see Expressions.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. If an array contains duplicate entries, $setDifference ignores the duplicate entries. $setDifference ignores the order of the elements.对数组执行集合操作，将数组视为集合。如果数组包含重复条目，$setDifference会忽略重复条目。$setDifference忽略元素的顺序。

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

If a set contains a nested array element, $setDifference does not descend into the nested array but evaluates the array at top-level.如果一个集合包含嵌套数组元素，则$setDifference不会下降到嵌套数组中，而是在顶级计算数组。

Example示例Result结果Notes备注
 
{ $setDifference: [
     [ "a", "c" ],
     [ "a", "b" ]
] }
 
[ "c" ]
"a" is present in both arrays, so it is removed from the result. "c" is only present in the first array."a"存在于两个数组中，因此它将从结果中删除。"c"仅存在于第一个数组中。
{ $setDifference: [
     [ "a", "c" ],
     [ "a", "b", "c" ]
] }
 
[ ]
Both "a" and "c" are present in both arrays so they are removed from the result. No more elements remain in the first array."a"和"c"都存在于两个数组中，因此它们会从结果中删除。第一个数组中不再有元素。
{ $setDifference: [
     [ "a", "b", "a" ],
     [ "b", "a" ]
] }
 
[ ]
Both "a" and "b" are present in both arrays so they are removed from the result. Duplicates are also removed. No more elements remain in the first array."a"和"b"都存在于两个数组中，因此它们会从结果中删除。重复项也会被删除。第一个数组中不再有元素。
{ $setDifference: [
     [ "a", "b" ],
     [ [ "a", "b" ] ]
] }
 
[ "a", "b" ]
The strings "a" and "b" from the first array are not found in the second array, which contains a nested array.在包含嵌套数组的第二个数组中找不到第一个数组中的字符串"a"和"b"。
{ $setDifference: [
     [ ],
     [ "a", "b" ]
] }
 
[ ]
The first array is empty so there are no elements to subtract from, regardless of what's in the second array.第一个数组是空的，因此无论第二个数组中有什么，都没有元素可以相减。
{ $setDifference: [
     [ "a", "a" ],
     [ "a", "b" ]
] }
 
[ ]
"a" is present in the second array so it is removed from the result. Duplicates are also removed. No more elements remain in the first array."a"存在于第二个数组中，因此将其从结果中删除。重复项也会被删除。第一个数组中不再有元素。

Example示例

Consider an flowers collection with the following documents:考虑一个有以下文件的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数组中但在flowerFieldA数组中找不到的元素数组：

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" ] }