Comparison expressions ($eq, $gt in aggregation) in MongoDB - Step-by-Step Execution

Concept Flow - Comparison expressions ($eq, $gt in aggregation)

Start aggregation pipeline

↓

Apply $match stage

↓

Evaluate $eq or $gt expression

↓

Filter documents based on comparison

↓

Output filtered documents

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End

The aggregation pipeline starts, applies a $match stage where $eq or $gt expressions compare fields to values, filters documents accordingly, and outputs the result.

Execution Sample

MongoDB

db.products.aggregate([
  { $match: { price: { $gt: 100 } } }
])

Filters products with price greater than 100 using $gt in aggregation.

Execution Table

Step	Document	Condition Evaluated	Result	Action
1	{_id:1, price:50}	price > 100	False	Exclude document
2	{_id:2, price:150}	price > 100	True	Include document
3	{_id:3, price:100}	price > 100	False	Exclude document
4	{_id:4, price:200}	price > 100	True	Include document
End	-	-	-	No more documents to evaluate

💡 All documents processed; only those with price > 100 included.

Variable Tracker

Variable	Start	After Doc 1	After Doc 2	After Doc 3	After Doc 4	Final
Included Documents	[]	[]	[{_id:2, price:150}]	[{_id:2, price:150}]	[{_id:2, price:150}, {_id:4, price:200}]	[{_id:2, price:150}, {_id:4, price:200}]

Key Moments - 2 Insights

Why is the document with price 100 excluded when using $gt: 100?

Because $gt means 'greater than', not 'greater than or equal to'. The execution_table row 3 shows price 100 evaluated as False.

How does $eq differ from $gt in filtering documents?

$eq checks for exact equality, while $gt checks if a value is strictly greater. The execution_table shows $gt filtering; $eq would include only documents matching exactly.

Visual Quiz - 3 Questions

Look at the execution_table, which document is included at step 2?

A {_id:2, price:150}

B {_id:1, price:50}

C {_id:3, price:100}

D {_id:4, price:200}

💡 Hint

Check the 'Result' and 'Action' columns at step 2 in execution_table.

At which step does the condition 'price > 100' become false for the first time?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Result' column in execution_table for step 1.

If we change $gt: 100 to $eq: 100, which document would be included?

A {_id:2, price:150}

B {_id:1, price:50}

C {_id:3, price:100}

D {_id:4, price:200}

💡 Hint

Refer to the document with price exactly 100 in execution_table.

Concept Snapshot

Aggregation $match uses comparison expressions like $eq and $gt.
$eq checks if a field equals a value.
$gt checks if a field is greater than a value.
Documents passing the condition are included in output.
Useful for filtering data in aggregation pipelines.

Full Transcript

This visual execution shows how MongoDB aggregation uses comparison expressions $eq and $gt inside a $match stage. Each document is checked against the condition, for example price > 100. Documents with price 150 and 200 pass and are included, while those with 50 and 100 do not. The variable tracker shows how the included documents list grows. Key moments clarify why 100 is excluded with $gt and how $eq differs. The quiz tests understanding of which documents pass and how changing the operator affects results.