Query filter syntax in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the field "age" is exactly 25.

MongoDB

db.collection.find({"age": [1] })

Drag options to blanks, or click blank then click option'

A25

B"age"

C{ $gt: 25 }

D[25]

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the number, which makes it a string.

Using an array instead of a single value.

Using a comparison operator when exact match is needed.

The filter {"age": 25} finds documents where the age field equals 25 exactly.

2fill in blank

medium

Complete the code to find documents where "score" is greater than 80.

MongoDB

db.collection.find({"score": { [1]: 80 }})

Drag options to blanks, or click blank then click option'

A$lt

B$eq

C$gt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$lt" which means less than.

Using "$eq" which means equal to.

Using "$ne" which means not equal.

The operator "$gt" means "greater than". So this filter finds documents with score > 80.

3fill in blank

hard

Fix the error in the query to find documents where "status" is not "active".

MongoDB

db.collection.find({"status": { [1]: "active" }})

Drag options to blanks, or click blank then click option'

A$ne

B$eq

C$gt

D$in

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$eq" which matches only "active" status.

Using "$gt" which is for greater than comparisons.

Using "$in" which expects an array of values.

The operator "$ne" means "not equal to". It filters documents where status is not "active".

4fill in blank

hard

Fill both blanks to find documents where "age" is at least 18 and less than 30.

MongoDB

db.collection.find({"age": { [1]: 18, [2]: 30 }})

Drag options to blanks, or click blank then click option'

A$gte

B$lt

C$lte

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$gt" instead of "$gte" excludes 18.

Using "$lte" includes 30 which is not wanted.

Using "$ne" which is unrelated.

"$gte" means "greater than or equal to" 18, and "$lt" means "less than" 30, so this finds ages between 18 and 29.

5fill in blank

hard

Fill all three blanks to find documents where "score" is greater than 50, "status" is "active", and "age" is less than 40.

MongoDB

db.collection.find({"score": { [1]: 50 }, "status": [2], "age": { [3]: 40 }})

Drag options to blanks, or click blank then click option'

A$gt

B"active"

C$lt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$ne" instead of "$gt" or "$lt".

Not quoting the string "active".

Mixing up greater than and less than operators.

"$gt" filters scores greater than 50, "status" must equal "active", and "$lt" filters ages less than 40.

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