分类查询
{ "_id" : ObjectId("515d8f53175b8ecb053425c2"), "category" : "Batteries", "products" : [ { "brand" : "Duracell", "item" : [ "AA", "AAA" ] }, { "brand" : "Everyday", "item" : [ "9V", "AA", "12V" ] } ]}{["AA", "AAA", "9V", "12V"]} 和 { "category" : "Batteries", "item": ["AA", "AAA", "9V", "12V"]}db.xyz.aggregate( {$project: {a: '$products.item'}}, {$unwind: '$a'}, {$unwind: '$a'}, {$group: {_id: 'a', items: {$addToSet: '$a'}}});db.xyz.aggregate( {$project: {category: 1, a: '$products.item'}}, {$unwind: '$a'}, {$unwind: '$a'}, {$group: {_id: '$category', items: {$addToSet: '$a'}}});
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