Combining comparison operators in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does combining comparison operators in MongoDB allow you to do?

It allows you to filter documents by applying multiple conditions on the same field, such as finding values greater than one number and less than another.

beginner

How do you combine comparison operators like $gt and $lt in a MongoDB query?

You put them inside the same field object, for example: { age: { $gt: 20, $lt: 30 } } to find ages greater than 20 and less than 30.

intermediate

What is the difference between using $and operator and combining comparison operators inside one field in MongoDB?

$and combines multiple conditions that can be on different fields, while combining comparison operators inside one field applies multiple conditions on the same field.

beginner

Example: How to find documents where price is between 10 and 50 in MongoDB?

Use: { price: { $gte: 10, $lte: 50 } } which means price greater than or equal to 10 and less than or equal to 50.

intermediate

Can you combine $ne (not equal) with other comparison operators on the same field in MongoDB?

Yes, for example: { score: { $ne: 100, $gt: 50 } } finds scores not equal to 100 and greater than 50.

How do you write a MongoDB query to find documents where age is greater than 18 and less than 30?

A { age: { $gt: 18 }, age: { $lt: 30 } }

B { age: { $gt: 18, $lt: 30 } }

C { $and: [ { age: { $gt: 18 } }, { age: { $lt: 30 } } ] }

D { age: { $gte: 18, $lte: 30 } }

Combining $gt and $lt inside the same field object is the correct syntax to find age greater than 18 and less than 30.

Which operator is used to combine multiple conditions on different fields in MongoDB?

A $lt

B $or

C $gt

D $and

$and is used to combine multiple conditions on different fields.

What does this query do? { price: { $gte: 100, $lte: 200 } }

A Finds documents with price exactly 100 or 200

B Finds documents with price less than 100 or greater than 200

C Finds documents with price between 100 and 200 inclusive

D Finds documents with price not equal to 100 and 200

$gte and $lte combined find values between 100 and 200 including those numbers.

Is this query valid? { score: { $ne: 50, $gt: 30 } }

A Yes, it finds scores not equal to 50 and greater than 30

B No, you cannot combine $ne with other operators

C No, $ne must be used alone

D Yes, but it finds scores equal to 50 or greater than 30

You can combine $ne with other comparison operators on the same field.

What is the difference between { age: { $gt: 20, $lt: 30 } } and { $and: [ { age: { $gt: 20 } }, { age: { $lt: 30 } } ] }?

A They are equivalent queries

B First is invalid syntax

C Second only works for different fields

D First only works for strings

Both queries find ages greater than 20 and less than 30; they are equivalent.

Explain how to combine multiple comparison operators on the same field in a MongoDB query.

Think about putting conditions inside one object for the field.

Describe the difference between using $and operator and combining comparison operators inside one field in MongoDB.

Consider when you want to filter by one field versus multiple fields.

Practice

1. Which MongoDB query correctly finds documents where the field age is greater than 20 and less than 30?

easy

A. { age: { $gte: 20, $lte: 30 } }

B. { age: { $gt: 20, $lt: 30 } }

C. { age: { $ne: 20, $ne: 30 } }

D. { age: { $gt: 20 }, $lt: 30 }

Solution

1. Step 1: Understand the requirement for age range

   The question asks for documents where age is strictly greater than 20 and strictly less than 30.

2. Step 2: Identify the correct MongoDB syntax for combined conditions

   Using { age: { $gt: 20, $lt: 30 } } combines both conditions on the same field correctly.

3. Final Answer:

   { age: { $gt: 20, $lt: 30 } } -> Option B

4. Quick Check:

   Combined $gt and $lt on same field = { age: { $gt: 20, $lt: 30 } } [OK]

Hint: Use $gt and $lt together inside one field object [OK]

Common Mistakes:

• Using $gte and $lte includes boundary values, not strictly between
• Placing $lt outside the field object causes syntax error
• Using $ne does not check range but excludes specific values

2. Which of the following is the correct syntax to find documents where score is not equal to 50 and less than or equal to 100?

easy

A. { score: { $ne: 50, $lte: 100 } }

B. { score: { $ne: 50 }, $lte: 100 }

C. { score: { $ne: 50, $lt: 100 } }

D. { score: { $ne: 50, $gte: 100 } }

Solution

1. Step 1: Combine $ne and $lte inside the same field object

   Both conditions must apply to score, so they go together inside one object.

2. Step 2: Check each option's syntax

   { score: { $ne: 50, $lte: 100 } } correctly combines $ne: 50 and $lte: 100 inside score. { score: { $ne: 50 }, $lte: 100 } separates $lte outside, which is invalid syntax.

3. Final Answer:

   { score: { $ne: 50, $lte: 100 } } -> Option A

4. Quick Check:

   Combine $ne and $lte inside one field object = { score: { $ne: 50, $lte: 100 } } [OK]

Hint: Put all comparison operators inside one field object [OK]

Common Mistakes:

• Placing $lte outside the field object causes syntax error
• Using $gte instead of $lte changes the condition
• Separating operators into different objects breaks query

3. Given the collection products with documents like { price: 150, stock: 30 }, what will the query { price: { $gt: 100, $lt: 200 }, stock: { $gte: 20, $lte: 40 } } return?

medium

A. Documents where price is greater than 100 or stock is less than 40

B. Documents where price is exactly 100 and stock is exactly 20

C. Documents where price is between 100 and 200, and stock is between 20 and 40

D. Documents where price is less than 100 and stock is greater than 40

Solution

1. Step 1: Understand the combined conditions on price and stock

   The query uses $gt and $lt on price to find values strictly between 100 and 200, and $gte and $lte on stock to find values between 20 and 40 inclusive.

2. Step 2: Interpret the logical AND behavior of MongoDB queries

   Both conditions must be true for a document to match, so only documents with price in (100,200) and stock in [20,40] are returned.

3. Final Answer:

   Documents where price is between 100 and 200, and stock is between 20 and 40 -> Option C

4. Quick Check:

   Combined conditions on fields = Documents where price is between 100 and 200, and stock is between 20 and 40 [OK]

Hint: All conditions on different fields combine with AND logic [OK]

Common Mistakes:

• Thinking conditions combine with OR instead of AND
• Confusing inclusive ($gte/$lte) with exclusive ($gt/$lt)
• Assuming exact matches instead of ranges

4. Identify the error in this MongoDB query: { quantity: { $gt: 10 }, $lt: 50 }

medium

A. The query is correct and will work as expected

B. The $gt operator should be $gte

C. The field name 'quantity' is missing

D. The $lt operator is outside the field object and causes syntax error

Solution

1. Step 1: Analyze the placement of comparison operators

   The $lt operator is placed outside the quantity field object, which is invalid syntax in MongoDB queries.

2. Step 2: Correct syntax requires all operators on the same field inside one object

   Both $gt and $lt must be inside the quantity object like { quantity: { $gt: 10, $lt: 50 } }.

3. Final Answer:

   The $lt operator is outside the field object and causes syntax error -> Option D

4. Quick Check:

   All operators must be inside field object = The $lt operator is outside the field object and causes syntax error [OK]

Hint: Keep all comparison operators inside the same field object [OK]

Common Mistakes:

• Placing operators outside the field object
• Confusing $gt with $gte unnecessarily
• Assuming query works despite syntax error

5. You want to find documents where rating is greater than 3 but not equal to 5, and reviews are between 10 and 100 inclusive. Which query correctly combines these conditions?

hard

A. { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } }

B. { rating: { $gt: 3 }, rating: { $ne: 5 }, reviews: { $gte: 10, $lte: 100 } }

C. { rating: { $gt: 3, $ne: 5, $gte: 10 }, reviews: { $lte: 100 } }

D. { rating: { $gt: 3, $ne: 5 }, reviews: { $gt: 10, $lt: 100 } }

Solution

1. Step 1: Combine multiple comparison operators on the same field correctly

   Both $gt: 3 and $ne: 5 must be inside the same rating object to apply both conditions.

2. Step 2: Use inclusive range operators for reviews

   Since reviews should be between 10 and 100 inclusive, use $gte: 10 and $lte: 100 inside the reviews object.

3. Step 3: Check for syntax errors and logical correctness

   { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } correctly combines all conditions. { rating: { $gt: 3 }, rating: { $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } repeats the rating field, which is invalid. { rating: { $gt: 3, $ne: 5, $gte: 10 }, reviews: { $lte: 100 } } incorrectly adds $gte: 10 to rating. { rating: { $gt: 3, $ne: 5 }, reviews: { $gt: 10, $lt: 100 } } uses exclusive operators for reviews, which is incorrect.

4. Final Answer:

   { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } -> Option A

5. Quick Check:

   Combine all conditions inside field objects correctly = { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } [OK]

Hint: Put all conditions for one field inside one object, use correct operators [OK]

Common Mistakes:

• Repeating the same field multiple times in query
• Mixing inclusive and exclusive operators incorrectly
• Adding unrelated operators inside wrong field objects