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

Concept Flow - $eq for equality

Start Query

↓

Check each document

↓

Compare field value with $eq value

↓

Include

↓

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.

Practice

1. What does the $eq operator do in a MongoDB query?

easy

A. It sorts documents in ascending order.

B. It deletes documents from the collection.

C. It updates documents with new values.

D. It matches documents where a field is equal to a specified value.

Solution

1. Step 1: Understand the purpose of $eq

   The $eq operator is used to filter documents where a field exactly matches a given value.

2. Step 2: Compare with other options

   Sorting, updating, and deleting are different operations and not related to $eq.

3. Final Answer:

   It matches documents where a field is equal to a specified value. -> Option D

4. Quick Check:

   $eq means equality match [OK]

Hint: Remember: $eq means 'equals' in queries [OK]

Common Mistakes:

• Confusing $eq with sorting or updating operators
• Thinking $eq modifies data instead of filtering
• Assuming $eq works for inequality

2. Which of the following is the correct syntax to find documents where the field age equals 30 using $eq?

easy

A. { age: { $eq: 30 } }

B. { $eq: { age: 30 } }

C. { age: $eq: 30 }

D. { age == 30 }

Solution

1. Step 1: Recall MongoDB query syntax for $eq

   The correct syntax uses the field name as key and an object with $eq as key and the value to match as value: { age: { $eq: 30 } }.

2. Step 2: Identify incorrect syntax

   Options B, C, and D do not follow MongoDB query syntax rules and will cause errors.

3. Final Answer:

   { age: { $eq: 30 } } -> Option A

4. Quick Check:

   Field: { $eq: value } format [OK]

Hint: Use { field: { $eq: value } } for equality [OK]

Common Mistakes:

• Placing $eq outside the field key
• Using double colons or wrong operators
• Using JavaScript equality syntax instead of MongoDB

3. Given the collection users with documents:

{ "name": "Alice", "age": 25 }, { "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }

What will the query db.users.find({ age: { $eq: 30 } }) return?

medium

A. [{ "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }]

B. [] (empty array)

C. [{ "name": "Alice", "age": 25 }]

D. Syntax error

Solution

1. Step 1: Understand the query condition

   The query looks for documents where age equals 30.

2. Step 2: Check documents matching the condition

   Bob and Carol have age 30, so both documents match and will be returned.

3. Final Answer:

   [{ "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }] -> Option A

4. Quick Check:

   age == 30 returns Bob and Carol [OK]

Hint: Look for documents where field equals value [OK]

Common Mistakes:

• Returning documents with age not equal to 30
• Expecting only one document instead of all matches
• Thinking $eq causes syntax error

4. You wrote the query db.products.find({ price: $eq: 100 }) but it gives an error. What is wrong?

medium

A. Field name price is invalid

B. Missing curly braces around $eq: 100 value

C. You cannot use $eq with numbers

D. The collection name products is incorrect

Solution

1. Step 1: Check the query syntax

   The $eq operator must be inside an object as the value for the field key, like { price: { $eq: 100 } }.

2. Step 2: Identify the missing braces

   The query is missing curly braces around $eq: 100, causing a syntax error.

3. Final Answer:

   Missing curly braces around $eq: 100 value -> Option B

4. Quick Check:

   Use { field: { $eq: value } } syntax [OK]

Hint: Always wrap $eq and value in braces { } [OK]

Common Mistakes:

• Omitting braces around $eq operator
• Assuming $eq works without object syntax
• Blaming collection or field name incorrectly

5. You want to find documents in orders where the status is exactly "shipped" and the quantity is 10. Which query correctly uses $eq for both conditions?

hard

A. db.orders.find({ status: "shipped", quantity: 10 })

B. db.orders.find({ $eq: { status: "shipped", quantity: 10 } })

C. db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } })

D. db.orders.find({ status == "shipped", quantity == 10 })

Solution

1. Step 1: Use $eq for each field separately

   Each field must have its own $eq operator inside an object: { status: { $eq: "shipped" } } and { quantity: { $eq: 10 } }.

2. Step 2: Combine conditions in one query object

   Put both conditions inside the find query object to filter documents matching both.

3. Final Answer:

   db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } }) -> Option C

4. Quick Check:

   Use separate $eq for each field [OK]

Hint: Use separate $eq for each field in the query [OK]

Common Mistakes:

• Trying to use one $eq for multiple fields
• Using JavaScript equality operators in query
• Omitting $eq and relying on implicit equality