# MongoDB UNION ALL 實現

### Version 4.4 or later 可使用 `$unionWith`

db.suppliers.insertMany([
  { _id: 1, supplier: "Aardvark and Sons", state: "Texas" },
  { _id: 2, supplier: "Bears Run Amok.", state: "Colorado"},
  { _id: 3, supplier: "Squid Mark Inc. ", state: "Rhode Island" },
])

db.warehouses.insertMany([
  { _id: 1, warehouse: "A", region: "West", state: "California" },
  { _id: 2, warehouse: "B", region: "Central", state: "Colorado"},
  { _id: 3, warehouse: "C", region: "East", state: "Florida" },
])

db.suppliers.aggregate([
   { $project: { state: 1, _id: 0 } },
   { $unionWith: { coll: "warehouses", pipeline: [ { $project: { state: 1, _id: 0 } } ]} }
])

### Before version 4.4 使用 `$lookup` 搭配 `$group.$push`, `$concatArrays`, `$unwind`, `$replaceRoot`

db.first.aggregate([{
    $group: { // create an array to hold all documents of the first collection
        "_id": null,
        "first": {
            $push: "$$ROOT"
        }
    }
}, {
    $lookup: { // perform some kind of ridiculous lookup which will return all documents from the second collection in an array
        from: "second",
        let: { /* we do not need any variables */ },
        pipeline: [ { $match: { /* this filter will match every document */ } } ],
        as: "second"
    }
}, {
    $project: {
        "all": { $concatArrays: [ "$first", "$second" ] } // merge the two collections
    }
}, {
    $unwind: "$all" // flatten the resulting array
}, {
    $replaceRoot: { "newRoot": "$all" } // move content of "all" field all the way up 
}], {
    allowDiskUse: true // make sure we do not run into memory issues
})

參考自：
[MongoDB Docs > $unionWith (aggregation)](https://www.mongodb.com/docs/v6.0/reference/operator/aggregation/unionWith/)
[stackoverflow > SQL 'UNION ALL' like implementation in MongoDB](https://stackoverflow.com/a/52728587/11563389)

###### tags: `MongoDB`