Comparison expressions ($eq, $gt in aggregation) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $eq operator do in MongoDB aggregation?

The $eq operator checks if two values are equal. It returns true if they are the same, otherwise false.

beginner

How does the $gt operator work in MongoDB aggregation?

The $gt operator checks if the first value is greater than the second value. It returns true if it is, otherwise false.

beginner

Write a simple MongoDB aggregation expression using $eq to check if the field status equals "active".

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

beginner

In aggregation, what type of value do $eq and $gt return?

Both $eq and $gt return a boolean value: true or false.

intermediate

Why are comparison expressions like $eq and $gt useful in MongoDB aggregation?

They help filter or create conditions inside aggregation pipelines, allowing you to compare values and decide which documents to include or how to transform data.

What does { $eq: ["$age", 30] } check in an aggregation pipeline?

A If the field <code>age</code> is greater than 30

B If the field <code>age</code> is not equal to 30

C If the field <code>age</code> is less than 30

D If the field <code>age</code> equals 30

The $eq operator checks if the first value equals the second. Here it checks if age is exactly 30.

Which operator would you use to check if a value is greater than another in MongoDB aggregation?

A $gt

B $ne

C $lt

D $eq

$gt means 'greater than'. It returns true if the first value is bigger than the second.

What type of value do $eq and $gt return in aggregation?

A Number

B String

C Boolean

D Array

Both $eq and $gt return true or false, which are boolean values.

In the expression { $gt: ["$score", 50] }, what does it mean if the result is true?

A The score equals 50

B The score is greater than 50

C The score is less than 50

D The score is not a number

$gt returns true if the first value is greater than the second. Here, score is greater than 50.

Which of these is a valid use of $eq in an aggregation pipeline?

A { $eq: ["$price", 100] }

B { $eq: "$price", 100 }

C { $eq: "$price" }

D { $eq: [100] }

The correct syntax for $eq is an array with two values to compare, like ["$price", 100].

Explain how the $eq and $gt operators work in MongoDB aggregation and give a simple example of each.

Think about comparing values and returning true or false.

Describe a real-life situation where you might use $eq and $gt in a MongoDB aggregation pipeline.

Imagine sorting or filtering data like a list of users or products.

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