The projection operator in MongoDB is used to include or exclude fields in the document that is returned from a query. This can be useful when you only need a subset of the data contained in a document.
Here is an example on how to use the projection operator:
Let’s say that we have a collection named “students” and each document in the students collection looks something like this:
```
{
“_id”: 1,
“name”: “John”,
“age”: 22,
“school”: “MIT”,
“score”: 88
}
```
To return only the name and age fields for each document in the collection, we would use the following command:
```
db.students.find({}, {name: 1, age: 1})
```
In this case, {} represents an empty query and returns all documents in the collection. The second parameter {name: 1, age: 1} is a projection operator that specifies the inclusion of the name and age fields in the returned documents.
To exclude certain fields, you can use the projection operator like this:
```
db.students.find({}, {school: 0, score: 0})
```
In this case, the school and score fields will be excluded from the returned documents. All other fields will be included.
Remember, you cannot mix inclusion and exclusion in the same projection, with the exception of the `_id` field. If you do not specify an `_id` field in the projection, MongoDB includes it in the result set. To exclude `_id` field from the result, add `_id: 0` in your projection.
```
db.students.find({}, {name: 1, age: 1, _id: 0})
```
In this case, only name and age are included in the result and \_id is excluded.
