A MongoDB document stores data in key-value pairs. When we delete a document, that record is removed from its collection.

Example document in the students collection:

{
    _id: 1,
    name: "Rahul",
    age: 20,
    course: "Java",
    fee: 5000
}

If Rahul leaves the course, we can delete his document using a suitable filter.

The deleteOne() method removes the first document that matches the specified filter.

Syntax

db.collection.deleteOne(
    { filter }
)

Example

db.students.deleteOne(
    { _id: 1 }
)

This command deletes the document whose _id is 1.

Delete a Student by Name

db.students.deleteOne(
    { name: "Rahul" }
)

If several documents have the name Rahul, this command removes only the first matching document.

Check the Remaining Documents

db.students.find()

The deleteMany() method removes all documents that match the specified filter.

Syntax

db.collection.deleteMany(
    { filter }
)

Example: Delete All Java Students

db.students.deleteMany(
    { course: "Java" }
)

Every document whose course is Java will be deleted from the students collection.

Example: Delete Students Below a Certain Age

db.students.deleteMany(
    { age: { $lt: 18 } }
)

This deletes all student documents whose age is less than 18.

We can use comparison operators to select documents for deletion.

Delete Products with a Price Below 100

db.products.deleteMany(
    { price: { $lt: 100 } }
)

Delete Employees with a Salary Above 100000

db.employees.deleteMany(
    { salary: { $gt: 100000 } }
)

Delete Inactive Users

db.users.deleteMany(
    { status: "inactive" }
)

The filter determines which documents are removed. Always verify the filter before running a delete command.

Passing an empty filter to deleteMany() removes every document from the collection.

db.students.deleteMany({})

The collection remains, but its documents are removed. This operation cannot be undone automatically.

To remove an entire collection, use the drop() method.

Example

db.students.drop()

This removes the students collection, including its documents and collection indexes.

Unlike deleteMany({}), which keeps the collection, drop() removes the collection itself.

• deleteOne() deletes at most one matching document.
• deleteMany() deletes every matching document.
• Use a specific filter when deleting individual records.
• deleteMany({}) deletes all documents in the collection.
• drop() removes the collection itself.
• Deletion commands do not provide a normal undo operation.
• Check important records before deleting them.

Create a collection named practice_delete and insert these sample documents:

db.practice_delete.insertMany([
    {
        _id: 1,
        name: "Rahul",
        age: 20,
        course: "Java"
    },
    {
        _id: 2,
        name: "Priya",
        age: 17,
        course: "Python"
    },
    {
        _id: 3,
        name: "Aman",
        age: 22,
        course: "Java"
    },
    {
        _id: 4,
        name: "Neha",
        age: 16,
        course: "HTML"
    }
])

Complete these tasks:

1. Display all documents using find().
2. Delete Rahul's document using deleteOne().
3. Delete all students whose age is less than 18.
4. Delete all students whose course is Java.
5. Display the remaining documents.
6. Explain the difference between deleteOne() and deleteMany().

Run the insertion command only once, or clear the test collection before inserting the same sample data again.