Combining comparison operators in MongoDB - Step-by-Step Execution

Concept Flow - Combining comparison operators

Start Query

↓

Specify Field

↓

Apply Comparison Operators

↓

Combine with $and / $or

↓

Execute Query

↓

Return Matching Documents

The query starts by specifying a field, applies comparison operators, combines them with logical operators like $and or $or, then executes to return matching documents.

Execution Sample

MongoDB

db.products.find({
  $and: [
    { price: { $gt: 10 } },
    { price: { $lt: 50 } }
  ]
})

This query finds products with price greater than 10 and less than 50.

Execution Table

Step	Query Part	Evaluation	Matching Documents
1	Start Query	Begin with empty filter	All documents
2	Apply { price: { $gt: 10 } }	Filter documents where price > 10	Docs with price > 10
3	Apply { price: { $lt: 50 } }	Filter documents where price < 50	Docs with price < 50
4	Combine with $and	Documents must satisfy both conditions	Docs with 10 < price < 50
5	Execute Query	Return filtered documents	Docs with price between 10 and 50
6	End	No more steps	Query complete

💡 Query ends after applying all conditions combined with $and

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Matching Documents	All documents	Docs with price > 10	Docs with price < 50	Docs with price > 10 and price < 50	Docs with price between 10 and 50

Key Moments - 2 Insights

Why do we need to use $and to combine multiple comparison operators on the same field?

Because MongoDB requires combining multiple conditions on the same field using $and; writing { price: { $gt: 10, $lt: 50 } } works too, but $and explicitly shows combining separate conditions. See execution_table rows 3 and 4.

Can we combine comparison operators without $and or $or?

Yes, if they are on the same field, you can combine them inside one object like { price: { $gt: 10, $lt: 50 } }. Using $and is necessary when combining conditions on different fields or complex logic. See execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution table, what documents remain after Step 3?

A Documents with price less than 50

B Documents with price greater than 10

C Documents with price greater than 10 and less than 50

D All documents

💡 Hint

Check the 'Matching Documents' column at Step 3 in the execution_table

At which step does the query combine conditions to require both to be true?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the step mentioning $and in the 'Query Part' column of execution_table

If we remove the $and and write { price: { $gt: 10, $lt: 50 } }, how does the variable 'Matching Documents' after Step 4 change?

A It includes documents with price less than 10

B It remains the same, documents with price between 10 and 50

C It includes documents with price greater than 50

D It becomes empty

💡 Hint

Refer to variable_tracker showing final matching documents for combined conditions on the same field

Concept Snapshot

MongoDB combines comparison operators using $and or $or.
Example: {$and: [{price: {$gt: 10}}, {price: {$lt: 50}}]}.
Alternatively, combine on same field: {price: {$gt: 10, $lt: 50}}.
Query returns documents matching all combined conditions.
Use $and for multiple fields or complex logic.

Full Transcript

This visual execution shows how MongoDB combines comparison operators to filter documents. The query starts empty, then applies price > 10, then price < 50, and combines these with $and to require both conditions. The matching documents narrow down step by step until only those with price between 10 and 50 remain. Key points include how $and combines conditions and that multiple operators on the same field can be combined inside one object. The execution table and variable tracker clearly show the filtering process and final results.

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