# NoSQL (MongoDB)
- ドキュメント指向
## Aggregation Pipeline （集計パイプライン）
- [Aggregation Pipeline - MongoDB Manual v8.0](https://www.mongodb.com/docs/manual/core/aggregation-pipeline/)
An aggregation pipeline consists of one or more [stages](https://www.mongodb.com/docs/manual/reference/operator/aggregation-pipeline/#std-label-aggregation-pipeline-operator-reference) that process documents:
- Each stage performs an operation on the input documents. For example, a stage can filter documents, group documents, and calculate values.
- The documents that are output from a stage are passed to the next stage.
- An aggregation pipeline can return results for groups of documents. For example, return the total, average, maximum, and minimum values.
## 例
```json
{
"member_id": 1,
"custom_fields": {
"職務スキル": "Java",
"プロジェクト経験": "3年以上"
}
}
```
## クロス集計
```js
db.members.aggregate([
{
$group: {
_id: {
skill: "$custom_fields.職務スキル",
experience: "$custom_fields.プロジェクト経験"
},
count: { $sum: 1 }
}
},
{
$sort: { "_id.skill": 1, "_id.experience": 1 }
}
])
```
| 職務スキル | プロジェクト経験 | count |
| ---------- | ---------------- | ----- |
| Java | 3年以上 | 5 |
| Java | 1-3年 | 8 |
| Python | 3年以上 | 4 |
| Python | 1-3年 | 6 |
### Pros and Cons
- **Pros**: 柔軟なスキーマ、`aggregation pipeline`による強力な集計機能、分散型スケーラビリティ
- **Cons**: ドキュメント内にデータを持つため、頻繁に更新がある場合は管理が複雑になる可能性