$eq for equality in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $eq operator do in MongoDB?

The $eq operator checks if a field's value is equal to a specified value. It returns documents where the field matches exactly.

beginner

How do you use $eq in a MongoDB query to find documents where age is 30?

You write: { age: { $eq: 30 } }. This finds documents where the age field equals 30.

intermediate

Is { age: 30 } the same as { age: { $eq: 30 } } in MongoDB queries?

Yes, both find documents where age equals 30. The first is shorthand; the second uses $eq explicitly.

intermediate

Can $eq be used to compare fields to null in MongoDB?

Yes, { field: { $eq: null } } finds documents where field is null or does not exist.

beginner

What is the difference between $eq and $ne in MongoDB?

$eq checks for equality (equal to), while $ne checks for inequality (not equal to).

What does { age: { $eq: 25 } } return in MongoDB?

A Documents where age is greater than 25

B Documents where age is not 25

C Documents where age is 25

D Documents where age is less than 25

The $eq operator matches documents where the field equals the specified value.

Is { name: { $eq: "Alice" } } equivalent to { name: "Alice" }?

A Yes, both find documents where name is Alice

B No, they find different documents

C Only the first works

D Only the second works

Both forms find documents where the name field equals "Alice".

Which operator finds documents where a field is NOT equal to a value?

A $eq

B $gt

C $lt

D $ne

$ne means 'not equal', opposite of $eq.

What happens if you query { score: { $eq: null } }?

A Finds documents where score is null or missing

B Finds documents where score is zero

C Finds documents where score is not null

D Returns an error

$eq: null matches fields that are null or do not exist.

Which is a correct way to find documents where status equals "active"?

A { status: "active" }

B Both B and C

C { status: { $eq: "active" } }

D Neither B nor C

Both forms are valid and find documents where status is "active".

Explain how the $eq operator works in MongoDB queries.

Think about how you find items that exactly match a value.

Describe the difference between using $eq and not using it in a MongoDB query.

Consider if <code>{ age: 30 }</code> and <code>{ age: { $eq: 30 } }</code> return the same documents.

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