Essential MongoDB Operations: A Guide to CRUD and Administration

MongoDB Operations

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1. How do you create a database in MongoDB?

In MongoDB, you don’t explicitly create a database. A database is created automatically when you first store data in it.

1. Switch to a database:

   use myDatabase

If the database does not exist, MongoDB creates it when you add data.

1. Verify creation: Add a collection and document, then use show dbs to list databases. Empty databases won’t appear.

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2. What is the insertOne method in MongoDB?

The insertOne method inserts a single document into a collection.

Syntax:

db.collection.insertOne(document)

Example:

db.users.insertOne({ name: "Alice", age: 25 })

• Adds a document to the users collection.
• Automatically adds an _id field if not provided.

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3. How do you insert multiple documents into a collection?

Use the insertMany method to add multiple documents.

Syntax:

db.collection.insertMany([document1, document2, ...])

Example:

db.users.insertMany([
  { name: "Bob", age: 30 },
  { name: "Charlie", age: 22 }
])

• Ensures all or none of the documents are inserted, depending on the write concern.

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4. Explain the find method in MongoDB.

The find method retrieves documents from a collection based on a query.

Syntax:

db.collection.find(query, projection)

• Query: Specifies the selection criteria.
• Projection: Specifies which fields to include/exclude.

Examples:

1. Find all documents:

   db.users.find({})

1. Find documents with criteria:

   db.users.find({ age: { $gt: 20 } })

1. Use projection to include only the name field:

   db.users.find({}, { name: 1, _id: 0 })

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5. What is the difference between findOne and find?

• findOne: Returns the first document that matches the query.
• find: Returns a cursor to all matching documents.

Examples:

1. findOne:

   db.users.findOne({ age: { $gt: 20 } })

• Returns the first document where age > 20.

1. find:

   db.users.find({ age: { $gt: 20 } })

• Returns all documents where age > 20.

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6. How do you update a document in MongoDB?

Use the updateOne or updateMany methods.

Syntax:

db.collection.updateOne(filter, update, options)
db.collection.updateMany(filter, update, options)

Example:

Update the age of a user named "Alice":

db.users.updateOne({ name: "Alice" }, { $set: { age: 26 } })

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7. What is the difference between updateOne and updateMany?

• updateOne: Updates the first document that matches the filter criteria.
• updateMany: Updates all documents that match the filter criteria.

Examples:

1. updateOne:

   db.users.updateOne({ age: { $gt: 20 } }, { $set: { verified: true } })

1. updateMany:

   db.users.updateMany({ age: { $gt: 20 } }, { $set: { verified: true } })

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8. How do you delete documents from a collection?

Use deleteOne or deleteMany to remove documents.

Syntax:

db.collection.deleteOne(filter)
db.collection.deleteMany(filter)

Example:

Delete all users with age > 30:

db.users.deleteMany({ age: { $gt: 30 } })

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9. Explain the deleteOne and deleteMany methods.

• deleteOne: Removes the first document that matches the filter criteria.
• deleteMany: Removes all matching documents.

Examples:

1. deleteOne:

   db.users.deleteOne({ name: "Alice" })

1. deleteMany:

   db.users.deleteMany({ age: { $gt: 25 } })

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10. What is the drop method used for?

The drop method is used to delete an entire collection or database.

Collection Drop:

db.collection.drop()

• Deletes all documents and the collection itself.

Database Drop:

db.dropDatabase()

• Deletes the database, including all its collections.

Use with caution, as dropped data cannot be recovered.

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These operations cover essential CRUD and administrative tasks in MongoDB.

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