Comparison expressions ($eq, $gt in aggregation) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match documents where the field 'score' equals 10.

MongoDB

{ $match: { score: { $[1]: 10 } } }

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

A$eq

B$ne

C$lt

D$gt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $eq will match values greater than 10, not equal.

Using $ne will match values not equal to 10.

The $eq operator checks if the field value is equal to the specified value.

2fill in blank

medium

Complete the code to match documents where the field 'age' is greater than 25.

MongoDB

{ $match: { age: { $[1]: 25 } } }

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

A$lt

B$gt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $eq will only match values equal to 25, not greater.

Using $lt will match values less than 25.

The $gt operator matches documents where the field value is greater than the specified value.

3fill in blank

hard

Fix the error in the code to correctly match documents where 'price' is greater than 100.

MongoDB

{ $match: { price: { $gt$[1] 100 } } }

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

A=

B,

C:

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas or equal signs instead of colon causes syntax errors.

In MongoDB queries, the operator and value must be separated by a colon :.

4fill in blank

hard

Fill both blanks to match documents where 'quantity' equals 5 and 'status' is greater than 2.

MongoDB

{ $match: { quantity: { $[1]: 5 }, status: { $[2]: 2 } } }

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

A$eq

B$gt

C$lt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the operators or using wrong ones like $lt or $ne.

$eq checks for equality and $gt checks for greater than.

5fill in blank

hard

Fill all three blanks to create a $match stage that finds documents where 'score' equals 90, 'age' is greater than 18, and 'level' equals 3.

MongoDB

{ $match: { score: { $[1]: 90 }, age: { $[2]: 18 }, level: { $[3]: 3 } } }

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

A$eq

B$gt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $ne instead of $eq or mixing up operators.

Use $eq for equality checks and $gt for greater than.

Practice

1. What does the $eq operator do in a MongoDB aggregation pipeline?

easy

A. Checks if one value is greater than another

B. Sorts documents in ascending order

C. Checks if two values are equal

D. Filters documents by existence of a field

Solution

1. Step 1: Understand the purpose of $eq

   The $eq operator compares two values to see if they are exactly the same.

2. Step 2: Compare with other operators

   Unlike $gt which checks if one value is greater, $eq checks equality.

3. Final Answer:

   Checks if two values are equal -> Option C

4. Quick Check:

   $eq means equal [OK]

Hint: Remember: eq means equal, gt means greater [OK]

Common Mistakes:

• Confusing $eq with $gt
• Thinking $eq sorts data
• Using $eq to check field existence

2. Which of the following is the correct syntax to use $gt in a MongoDB aggregation $match stage to find documents where age is greater than 30?

easy

A. { $match: { $gt: { age: 30 } } }

B. { $match: { age: { $gt: 30 } } }

C. { $match: { age: $gt(30) } }

D. { $match: { age: { gt: 30 } } }

Solution

1. Step 1: Recall correct operator syntax

   MongoDB operators like $gt are used as keys inside an object with the field name as the key.

2. Step 2: Match correct structure

   The correct syntax is { field: { $gt: value } }, so { age: { $gt: 30 } } is correct.

3. Final Answer:

   { $match: { age: { $gt: 30 } } } -> Option B

4. Quick Check:

   Correct operator syntax = { $match: { age: { $gt: 30 } } } [OK]

Hint: Operators go inside field object with $ prefix [OK]

Common Mistakes:

• Placing $gt outside the field object
• Using parentheses like a function
• Missing $ before gt

3. Given the aggregation pipeline stage:

{ $match: { score: { $gt: 80 } } }

Which documents will be matched if the collection has documents with scores: 75, 80, 85, 90?

medium

A. Documents with scores 75, 85, and 90

B. Documents with scores 75 and 80

C. Documents with scores 80, 85, and 90

D. Documents with scores 85 and 90

Solution

1. Step 1: Understand $gt meaning

   The $gt operator matches values strictly greater than the given number, here 80.

2. Step 2: Check which scores are greater than 80

   Scores 85 and 90 are greater than 80; 75 and 80 are not.

3. Final Answer:

   Documents with scores 85 and 90 -> Option D

4. Quick Check:

   Only scores > 80 match = Documents with scores 85 and 90 [OK]

Hint: Remember $gt excludes equal values [OK]

Common Mistakes:

• Including values equal to 80
• Confusing $gt with $gte
• Selecting lower values by mistake

4. Identify the error in this aggregation $match stage:

{ $match: { $eq: ["$status", "active"] } }

medium

A. Incorrect use of $eq outside a field key

B. Missing $gt operator

C. Wrong array syntax for $eq

D. No error, syntax is correct

Solution

1. Step 1: Check $eq usage in $match

   The $eq operator is usually used inside $expr or aggregation expressions, not directly as a key in $match.

2. Step 2: Understand correct placement

   To compare fields with $eq, it must be inside $expr, like { $match: { $expr: { $eq: ["$status", "active"] } } }.

3. Final Answer:

   Incorrect use of $eq outside a field key -> Option A

4. Quick Check:

   $eq needs $expr for expressions [OK]

Hint: Use $expr to compare fields with $eq [OK]

Common Mistakes:

• Using $eq directly in $match without $expr
• Confusing $eq array syntax
• Assuming $eq works like a filter key

5. You want to filter documents where price is greater than 100 and category equals "electronics" using aggregation. Which $match stage correctly uses $gt and $eq together?

hard

A. { $match: { price: { $gt: 100 }, category: "electronics" } }

B. { $match: { $and: [ { price: { $gt: 100 } }, { category: { eq: "electronics" } } ] } }

C. { $match: { price: { $gt: 100 }, category: { eq: "electronics" } } }

D. { $match: { $expr: { $and: [ { gt: ["$price", 100] }, { eq: ["$category", "electronics"] } ] } } }

Solution

1. Step 1: Understand simple field comparisons

   For simple equality, you can directly write category: "electronics" without using $eq.

2. Step 2: Combine conditions in $match

   MongoDB $match supports multiple field conditions combined with AND by default, so { price: { $gt: 100 }, category: "electronics" } works.

3. Step 3: Check other options

   { $match: { price: { $gt: 100 }, category: { $eq: "electronics" } } } misuses $eq inside $match (not needed). { $match: { $and: [ { price: { $gt: 100 } }, { category: { $eq: "electronics" } } ] } } uses $and unnecessarily. { $match: { $expr: { $and: [ { $gt: ["$price", 100] }, { $eq: ["$category", "electronics"] } ] } } } uses $expr which is more complex than needed.

4. Final Answer:

   { $match: { price: { $gt: 100 }, category: "electronics" } } -> Option A

5. Quick Check:

   Simple conditions combine directly in $match [OK]

Hint: Use direct equality for fields, $gt for ranges [OK]

Common Mistakes:

• Overusing $eq inside $match
• Using $and unnecessarily
• Using $expr when not needed