Query filter syntax in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a query filter in MongoDB?

A query filter is a way to specify conditions to find documents in a collection that match certain criteria.

beginner

How do you filter documents where the field 'age' is exactly 25?

Use { "age": 25 } as the filter to find documents where the 'age' field equals 25.

beginner

What operator do you use to find documents where a field is greater than a value?

Use the $gt operator. For example, { "age": { "$gt": 30 } } finds documents where 'age' is greater than 30.

intermediate

How do you combine multiple conditions in a MongoDB query filter?

By default, listing multiple fields means AND. For OR, use the $or operator with an array of conditions.

intermediate

What does this filter do? { "status": { "$in": ["A", "B"] } }

It finds documents where the 'status' field is either 'A' or 'B'. The $in operator checks if the field matches any value in the array.

Which filter finds documents where 'score' is less than 50?

A { "score": { "$eq": 50 } }

B { "score": { "$gt": 50 } }

C { "score": 50 }

D { "score": { "$lt": 50 } }

The $lt operator means 'less than', so { "score": { "$lt": 50 } } finds scores below 50.

How do you find documents where 'age' is 20 AND 'status' is 'active'?

A { "$or": [ { "age": 20 }, { "status": "active" } ] }

B { "age": 20, "status": "active" }

C { "age": { "$eq": 20 }, "status": { "$eq": "active" } }

D { "$and": [ { "age": 20 }, { "status": "active" } ] }

Listing multiple fields in the filter means AND by default, so option B is correct. Option D is also correct as it explicitly uses $and.

What does the $ne operator do?

A Matches values greater than the specified value

B Matches values equal to the specified value

C Matches values not equal to the specified value

D Matches values less than the specified value

$ne means 'not equal', so it finds documents where the field is not the given value.

Which filter finds documents where 'category' is either 'books' or 'electronics'?

A { "category": { "$in": ["books", "electronics"] } }

B { "category": { "$all": ["books", "electronics"] } }

C { "category": "books" }

D { "$or": [ { "category": "books" }, { "category": "electronics" } ] }

$in matches any value in the array, so option A finds documents with either category. Option D also finds documents with either category using $or.

How do you find documents where 'price' is between 10 and 20 inclusive?

A { "price": { "$gte": 10, "$lte": 20 } }

B { "price": { "$gt": 10, "$lt": 20 } }

C { "price": { "$in": [10, 20] } }

D { "price": { "$ne": 10, "$ne": 20 } }

$gte means 'greater than or equal', $lte means 'less than or equal', so option D finds prices between 10 and 20.

Explain how to write a MongoDB query filter to find documents where a field matches multiple conditions.

Think about combining conditions with AND and OR.

Describe the purpose and usage of the $in operator in MongoDB query filters.

It helps find documents matching any value from a set.

Practice

1. What does a MongoDB query filter do?
{ age: 25 }

easy

A. Deletes documents where age is 25

B. Updates documents to set age to 25

C. Selects documents where the age field is exactly 25

D. Creates a new document with age 25

Solution

1. Step 1: Understand the role of a query filter

   A query filter in MongoDB is used to find documents that match certain conditions, not to modify or delete them.

2. Step 2: Analyze the filter { age: 25 }

   This filter matches documents where the field age has the value 25 exactly.

3. Final Answer:

   Selects documents where the age field is exactly 25 -> Option C

4. Quick Check:

   Query filter = select matching documents [OK]

Hint: Filters pick matching data, not modify or delete [OK]

Common Mistakes:

• Confusing filter with update or delete commands
• Thinking filter creates or changes documents
• Assuming filter matches partial or range values without operators

2. Which of these is the correct syntax to find documents where score is greater than 80?

easy

A. { score > 80 }

B. { score: gt 80 }

C. { $score: { gt: 80 } }

D. { score: { $gt: 80 } }

Solution

1. Step 1: Recall MongoDB operator syntax

   MongoDB uses JSON objects with operators starting with a dollar sign, like $gt for 'greater than'.

2. Step 2: Check each option

   { score: { $gt: 80 } } uses { score: { $gt: 80 } } which is correct syntax. Others miss the dollar sign or use invalid syntax.

3. Final Answer:

   { score: { $gt: 80 } } -> Option D

4. Quick Check:

   Use $gt for greater than in filters [OK]

Hint: Operators start with $ inside field object [OK]

Common Mistakes:

• Omitting $ before operator
• Using comparison operators like > directly
• Wrong operator names without $

3. Given the collection documents:

{ name: "Alice", age: 30 }
{ name: "Bob", age: 25 }
{ name: "Carol", age: 30 }

What will this query return?
db.collection.find({ age: 30 })

medium

A. [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }]

B. [{ name: "Bob", age: 25 }]

C. [] (empty array)

D. Syntax error

Solution

1. Step 1: Understand the filter condition

   The filter { age: 30 } matches documents where the age field equals 30.

2. Step 2: Identify matching documents

   Documents for Alice and Carol have age 30, so both match. Bob has age 25, so does not match.

3. Final Answer:

   [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }] -> Option A

4. Quick Check:

   Filter matches exact age 30 documents [OK]

Hint: Filter matches exact field values without operators [OK]

Common Mistakes:

• Expecting only one document returned
• Confusing filter with update or delete
• Thinking filter matches partial or range without operators

4. What is wrong with this MongoDB query filter?
db.collection.find({ age: $gt: 20 })

medium

A. Using $gt instead of $gte

B. Missing curly braces around $gt operator

C. Field name should be $age, not age

D. No error, query is correct

Solution

1. Step 1: Check operator syntax in filter

   MongoDB requires operators like $gt to be inside an object as a value for the field key.

2. Step 2: Identify the syntax error

   The query uses { age: $gt: 20 } which is invalid. It should be { age: { $gt: 20 } } with curly braces around $gt: 20.

3. Final Answer:

   Missing curly braces around $gt operator -> Option B

4. Quick Check:

   Operators must be inside braces as field values [OK]

Hint: Put operators inside braces after field name [OK]

Common Mistakes:

• Omitting braces around operator
• Using wrong operator names
• Misplacing $ sign

5. You want to find documents where status is either "active" or "pending". Which filter correctly does this?

hard

A. { status: { $in: ["active", "pending"] } }

B. { status: { $or: ["active", "pending"] } }

C. { $or: { status: "active", status: "pending" } }

D. { status: "active" || "pending" }

Solution

1. Step 1: Understand how to match multiple values

   MongoDB uses $in operator to match a field against any value in a list.

2. Step 2: Analyze each option

   { status: { $in: ["active", "pending"] } } uses { status: { $in: ["active", "pending"] } } which is correct. { status: { $or: ["active", "pending"] } } uses invalid operator $or inside field. { $or: { status: "active", status: "pending" } } has wrong syntax for $or. { status: "active" || "pending" } uses invalid JavaScript syntax.

3. Final Answer:

   { status: { $in: ["active", "pending"] } } -> Option A

4. Quick Check:

   Use $in for matching any value in list [OK]

Hint: Use $in with array for multiple value matches [OK]

Common Mistakes:

• Using $or inside field instead of top-level
• Wrong syntax for $or operator
• Using JavaScript operators inside filter