Why logical operators matter in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where age is greater than 25.

MongoDB

db.users.find({ age: { [1]: 25 } })

Drag options to blanks, or click blank then click option'

A$lt

B$gt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt will find ages less than 25, which is wrong here.

Using $eq looks for exactly 25, not greater than 25.

The $gt operator means 'greater than'. It finds documents where the age is more than 25.

2fill in blank

medium

Complete the code to find documents where status is either 'A' or 'B'.

MongoDB

db.orders.find({ [1]: [ { status: 'A' }, { status: 'B' } ] })

Drag options to blanks, or click blank then click option'

A$or

B$not

C$and

D$nor

Attempts:

3 left

💡 Hint

Common Mistakes

Using $and requires both conditions to be true, which is too strict here.

Using $not negates a condition, which is not what we want.

The $or operator finds documents matching any of the conditions inside the array.

3fill in blank

hard

Fix the error in the query to find documents where age is greater than 20 and less than 30.

MongoDB

db.people.find({ [1]: [ { age: { $gt: 20 } }, { age: { $lt: 30 } } ] })

Drag options to blanks, or click blank then click option'

A$not

B$or

C$nor

D$and

Attempts:

3 left

💡 Hint

Common Mistakes

Using $or would find ages greater than 20 or less than 30, which is too broad.

Using $not or $nor changes the meaning to exclude conditions.

The $and operator ensures both conditions are true: age greater than 20 and less than 30.

4fill in blank

hard

Fill both blanks to find documents where score is greater than 80 and status is 'active'.

MongoDB

db.results.find({ [1]: [ { score: { $gt: 80 } }, { status: [2] } ] })

Drag options to blanks, or click blank then click option'

A$and

B'active'

C'inactive'

D$or

Attempts:

3 left

💡 Hint

Common Mistakes

Using $or would allow either condition, not both.

Using 'inactive' for status would find wrong documents.

Use $and to combine conditions and the string 'active' for the status value.

5fill in blank

hard

Fill all three blanks to find documents where age is between 18 and 25 inclusive, and status is not 'inactive'.

MongoDB

db.users.find({ [1]: [ { age: { [2]: 18 } }, { age: { [3]: 25 } }, { status: { $ne: 'inactive' } } ] })

Drag options to blanks, or click blank then click option'

A$and

B$gte

C$lte

D$or

Attempts:

3 left

💡 Hint

Common Mistakes

Using $or would allow any condition, not all.

Using $gt or $lt excludes the boundary values.

Use $and to combine conditions, $gte for 'greater than or equal to' 18, and $lte for 'less than or equal to' 25.

Practice

1. Which logical operator in MongoDB requires all conditions to be true for a document to match?

easy

A. $and

B. $or

C. $not

D. $nor

Solution

1. Step 1: Recall MongoDB logical operators

   $and combines multiple conditions and only matches documents where every condition is true. $or matches if any condition is true, $not excludes matches, and $nor matches if none are true.

2. Final Answer:

   $and -> Option A

3. Quick Check:

   $and requires all true [OK]

Hint: Remember $and means all conditions must be true [OK]

Common Mistakes:

• Confusing $and with $or
• Thinking $not means all true
• Mixing $nor with $and

2. Which of the following is the correct syntax to find documents where age is greater than 25 or status is "active" in MongoDB?

easy

A. { $and: [ { age: { $gt: 25 } }, { status: "active" } ] }

B. { $nor: [ { age: { $gt: 25 } }, { status: "active" } ] }

C. { $not: [ { age: { $gt: 25 } }, { status: "active" } ] }

D. { $or: [ { age: { $gt: 25 } }, { status: "active" } ] }

Solution

1. Step 1: Identify $or operator and verify syntax

   $or matches documents where at least one condition is true, perfect for age > 25 or status = "active". The syntax with $or and array of conditions is correct; $and requires both true, $not and $nor have different meanings.

2. Final Answer:

   { $or: [ { age: { $gt: 25 } }, { status: "active" } ] } -> Option D

3. Quick Check:

   $or for either condition true = { $or: [ { age: { $gt: 25 } }, { status: "active" } ] } [OK]

Hint: Use $or for any condition true, syntax needs array [OK]

Common Mistakes:

• Using $and instead of $or
• Wrong array brackets
• Using $not with array incorrectly

3. Given the collection documents:
{ name: "Alice", age: 30, status: "active" }
{ name: "Bob", age: 20, status: "inactive" }
{ name: "Carol", age: 25, status: "active" }
What will the query { $and: [ { age: { $gt: 20 } }, { status: "active" } ] } return?

medium

A. [{ name: "Bob", age: 20, status: "inactive" }]

B. [{ name: "Alice", age: 30, status: "active" }, { name: "Carol", age: 25, status: "active" }]

C. [{ name: "Alice", age: 30, status: "active" }]

D. []

Solution

1. Step 1: Evaluate documents against $and conditions

   age > 20 and status = "active": Alice (30, active) matches, Carol (25, active) matches, Bob (20, inactive) does not. Both Alice and Carol returned.

2. Final Answer:

   [{ name: "Alice", age: 30, status: "active" }, { name: "Carol", age: 25, status: "active" }] -> Option B

3. Quick Check:

   Both age > 20 and active status = [{ name: "Alice", age: 30, status: "active" }, { name: "Carol", age: 25, status: "active" }] [OK]

Hint: Check each condition carefully for all documents [OK]

Common Mistakes:

• Including Bob who fails conditions
• Returning only one document
• Confusing $and with $or results

4. You wrote this query to exclude documents where status is "inactive":
{ $not: { status: "inactive" } }
But it returns an error. What is the problem?

medium

A. $not must be inside $and

B. $not cannot be used with strings

C. $not requires a condition operator like $eq, not a direct value

D. $not only works with numeric fields

Solution

1. Step 1: Identify $not syntax error

   $not expects a condition operator like { status: { $not: { $eq: "inactive" } } }, not { $not: { status: "inactive" } } with direct value.

2. Final Answer:

   $not requires a condition operator like $eq, not a direct value -> Option C

3. Quick Check:

   $not needs operator condition [OK]

Hint: Use $not with operators like $eq, not direct values [OK]

Common Mistakes:

• Passing direct values to $not
• Assuming $not works alone
• Using $not with arrays incorrectly

5. You want to find documents where age is greater than 20 but NOT with status equal to "inactive". Which query correctly uses logical operators to achieve this?

hard

A. { $and: [ { age: { $gt: 20 } }, { status: { $ne: "inactive" } } ] }

B. { $and: [ { age: { $gt: 20 } }, { $not: { status: { $eq: "inactive" } } } ] }

C. { $or: [ { age: { $gt: 20 } }, { $not: { status: { $eq: "inactive" } } } ] }

D. { $and: [ { age: { $gt: 20 } }, { $not: { status: "inactive" } } ] }

Solution

1. Step 1: Analyze requirement and options

   Need age > 20 AND status != "inactive". A: $not with direct value (invalid). B: $not wrongly placed at top-level; must be { status: { $not: { $eq: "inactive" } } } (but $ne simpler). C: $or allows documents failing one condition. D: Correctly uses $and with $gt and $ne.

2. Final Answer:

   { $and: [ { age: { $gt: 20 } }, { status: { $ne: "inactive" } } ] } -> Option A

3. Quick Check:

   Use $ne for NOT equal, combine with $and = { $and: [ { age: { $gt: 20 } }, { status: { $ne: "inactive" } } ] } [OK]

Hint: Use $ne for NOT equal, combine with $and for all true [OK]

Common Mistakes:

• Using $not with direct value
• Using $or instead of $and
• Overcomplicating with $not and $eq