Lecture 12- MongoDB Flashcards
What is a non-relational database?
- Polymorphic data structure
- Flexible schemas
- Easy to scale large workloads
Describe the document model
- JSON notation
- Field-value pairs are separated by colon
- Fields must be enclosed within quotation marks (string values are often quoted as well)
- Two field-values are separated by commas
What is a document?
A way to organize and store data as a set of field-value pairs in MongoDB
What is a collection?
An organized store of documents in MongoDB, usually with common fields between documents
What is the different between these two documents?
Two documents in the same collection but with different fields
Whats the difference between tabular (relational) data model vs document data model?
- Tabular = related data is split across multiple records and tables
- Document = related data is contained in a single, rich document
Describe MongoDB query language
- simple syntax
- designed to query documents
- only queries a single collection
What is this query doing?
Find documents (in people collection) with the age field value $lt (less than) 30
What is this query doing?
Insert some real data on people
What is this query doing?
Find people whose age is below 30
How do you use MQL find?
What is insertOne(), insertMany(), writeConcern and ordered?
- insertOne() = insert one document into a collection
- insertMany() = insert an array of documents into a collection
- writeConcern = sets the level of acknowledgement requested from MongoDB for write operations
- ordered = for insertMany() there is an additional option for controlling whether the documents are inserted in ordered or unordered fashion
Give an example using insertOne(), insertMany(), writeConcern and ordered
What is updateOne() and updateMany()?
- updateOne() = update one document into a collection
- updateMany() = update an array of documents into a collection
Give an example using updateOne() and updateMany()
What is deleteOne(), deleteMany() and writeConcern?
- deleteOne() = deletes one document from a collection
- deleteMany() = deletes many documents from a collection
- writeConcern = sets the level of acknowledgement requested from MongoDB for write operations
Give an example using deleteOne(), deleteMany() and writeConcern
What are some MQL query operator comparisons and what they do?
- $lt = exists and less than
- $lte = exists and less than or equal to
- $gt = exists and greater than
- $gte = exists and greater than or equal to
- $ne = does not exist or does not equal to
- $eq = exists and equal to
- $in = exists and in a set
- $nin = does not exists or not in a set
- $or = match either of two or more values
- $not = used with other operators to negate
- $nor = match neither of two or more values
- $and = match both of two or more values
Give an example using $and
Give an example using $not
Can MongoDB operators be combined?
Yes, allows for more complex conditions to be included in the query
AND and OR are what type of operators?
Boolean operators
Give an example of a query using an implicit AND and OR
Give an example of a query using explicit AND and OR
Give an example of a query using the NOT operator
What is the format of aggregation?
Describe MongoDB aggregation framework
- Complex operations broken into stages
- Operators called within stages
- Functionality within the DB
What is the MQL find() equivalent to $match, $projection, $sort, $limit, $skip and $count?
Complete this task using find() and aggregate(): run a query to find the first ten documents for farm ‘1’
An aggregation pipeline is an ____ and is used to hold the stages to execute and the parameters for that stage
Array
Each stage is a ____
Document (JSON like)
What does $match, $project and $limit do?
- $match = filters documents based on conditions
- $project = selects specific fields to display
- $limit =restricts the number of results returned
Describe what is going on in this query
- { $match: { “farm”: 1 } }: document format, filters the documents to include only those where farm is equal to 1
- { $project: { “name”: 1, “milk”: 1, “-id”: 0 } } document format, reshapes each document by specifying which fields to include or exclude, “name”: 1 and “milk”: 1 mean to include the name and milk fields, * “-id”: 0 means to execute the -id field
- { $limit: 10 }: limits the result set to a maximum of 10 documents
What does $group do?
This stage takes the incoming stream of documents, and segments it (each group is represented by a single document)
Complete this task: find the total marks for each student across all subjects
Complete this task: find the maximum marks scored in each subject
What is this query doing?
- Find the top two subjects based on average marks
- Finding the average marks per students
- Sorting the output based on average marks in descending order
- Limiting the output to two documents
What does $addFields do?
This stage takes the incoming stream of documents and adds a new field to the document as it is processed
What is this query doing?
- A new field called age is being added to each document
- Age is determined by the JavaScript function defined in the $function operator
- Math.random() = generates a random number between 0 (inclusive) and 1 (exclusive)
- Math.random() * 5 = scales the random number to a range between 0 and 5
- (Math.random() * 5) +1 = adding 1 shifts the range to 1 to 6 (exclusive of 6)
- Math.floor(…) = this rounds down the random number to the nearest whole number, giving an integer value between 1 and 5
- The function generates a random integer between 1 and 5, which will be the value of age for each document
What are the three main parameters for $function?
- Body = code that defined the function, generates a random number an assigns it to the age field
- Args = array of arguments that can be passed to the function, empty
- Lang = the language to use for the function, JavaScript
What does $project do?
$project stage transforms the documents by specifying which fields to include or modify
What is going on in this query?
- “expense”: 1 (includes the field expense in the output)
- “airline”: “$details.airline” (includes a field named airline, with its value coming from the details.airline field in the original document
- “flight_class”: “$details.seat_class” = includes a field called flight_class, using the value from details.seat_class
- “flight_type”:”$details.flight_type” = includes a field called flight_type, with its value from details.flight_type
- overspend: { $subtract: [ “$spent”, “$budget” ] } = creates a new field called overspend by calculating the difference between spent and budget
- $subtract operator that performs subtraction between two values
What does $unwind do?
- Deconstruct an array field outputing a document for each element
- It deconstructs an array field to output a document for each element in that array
- This means that each output document represents one of the array field values and each value will have its own document
- It essentially splits a document that contains an array into multiple documents, each with a single value from the array
- Specify a field path to indicate the array to be deconstructed or specify a document operator
- Unwind nested arrays
- Control ouput documents if there are empty or null arrays
- Handling empty or missing arrays
Give an example of a query using $unwind
What does the $lookup stage do?
- Performs a left outer join between two collections
- Used to combine data from multiple collections based on a specific relationship
- References documents from one collection within another collection
What is going on in this query?
- From = the name of the other collection you want to join with
- LocalField = the field from the current collection that you want to match with localField
- ForeignField = the field from the collection that you want to match with localField
- As = the name of the new array field where the matching documents form the collection will be sorted
What is this query doing?
Find the orders made by each customer
