$eq for equality in MongoDB - Step-by-Step Execution

Concept Flow - $eq for equality

Start Query

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Check each document

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Compare field value with $eq value

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Include

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Return matched documents

The query checks each document's field value against the $eq value. If equal, the document is included in the result.

Execution Sample

MongoDB

db.collection.find({ age: { $eq: 30 } })

Finds all documents where the age field equals 30.

Execution Table

Step	Document	Field Value (age)	Condition (age == 30)	Result
1	{ name: 'Alice', age: 25 }	25	25 == 30? False	Exclude
2	{ name: 'Bob', age: 30 }	30	30 == 30? True	Include
3	{ name: 'Carol', age: 35 }	35	35 == 30? False	Exclude
4	{ name: 'Dave', age: 30 }	30	30 == 30? True	Include
5	No more documents	-	-	Stop

💡 All documents checked, query returns only those with age equal to 30.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current Document	None	{ name: 'Alice', age: 25 }	{ name: 'Bob', age: 30 }	{ name: 'Carol', age: 35 }	{ name: 'Dave', age: 30 }	None
age	N/A	25	30	35	30	N/A
Condition Result	N/A	False	True	False	True	N/A
Included in Result	N/A	No	Yes	No	Yes	N/A

Key Moments - 2 Insights

Why does the document with age 25 get excluded even though it has an age field?

Because the $eq operator checks for exact equality. In the execution_table row 1, 25 == 30 is False, so the document is excluded.

What happens if a document does not have the age field?

It will be excluded because the field value is undefined and does not equal 30, similar to the False condition in the execution_table.

Visual Quiz - 3 Questions

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

A { name: 'Bob', age: 30 }

B { name: 'Alice', age: 25 }

C { name: 'Carol', age: 35 }

D { name: 'Dave', age: 30 }

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the condition age == 30 become false for the first time?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

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

If the query was changed to { age: { $eq: 35 } }, which document would be included at step 3?

A { name: 'Alice', age: 25 }

B { name: 'Bob', age: 30 }

C { name: 'Carol', age: 35 }

D { name: 'Dave', age: 30 }

💡 Hint

Refer to the 'Field Value' and 'Condition' columns in the execution_table at step 3.

Concept Snapshot

$eq operator checks if a field's value equals a specified value.
Syntax: { field: { $eq: value } }
Returns documents where field == value.
Useful for exact matches in queries.
If field missing or not equal, document excluded.

Full Transcript

The $eq operator in MongoDB queries checks if a document's field value exactly matches a given value. The query examines each document one by one. For each document, it compares the field's value to the $eq value. If they are equal, the document is included in the results; otherwise, it is excluded. For example, querying { age: { $eq: 30 } } returns only documents where the age field is exactly 30. Documents with different ages or missing the age field are not included. This step-by-step process ensures only matching documents are returned.