Comparison operators (==, <, >, >=, <=) in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Firebase Realtime Database: Query with < operator

You want to retrieve all users whose age is less than 30 from your Firebase Realtime Database. Which query will correctly return these users?

Firebase

firebase.database().ref('users').orderByChild('age').<operator>(30).once('value').then(snapshot => { /* handle data */ });

A firebase.database().ref('users').orderByChild('age').endBefore(30).once('value')

B firebase.database().ref('users').orderByChild('age').startAt(30).once('value')

C firebase.database().ref('users').orderByChild('age').equalTo(30).once('value')

D firebase.database().ref('users').orderByChild('age').endAt(29).once('value')

💡 Hint

Think about which method limits results to values less than a given number.

In Firebase Realtime Database, endBefore(30) returns all records with 'age' less than 30. startAt(30) returns ages 30 and above. equalTo(30) returns only age 30. endAt(29) returns ages up to 29 inclusive, but 29 is less than 30, so it works similarly for integer ages.

❓ Architecture

intermediate

Firestore: Filtering documents with >= operator

You want to get all orders with a total amount greater than or equal to 100 in Firestore. Which query correctly achieves this?

Firebase

firebase.firestore().collection('orders').where('total', '<operator>', 100).get().then(snapshot => { /* handle data */ });

A firebase.firestore().collection('orders').where('total', '>=', 100).get()

B firebase.firestore().collection('orders').where('total', '>', 100).get()

C firebase.firestore().collection('orders').where('total', '<=', 100).get()

D firebase.firestore().collection('orders').where('total', '==', 100).get()

💡 Hint

Remember that >= means greater than or equal to.

The 'where' clause with '>=' returns documents where 'total' is 100 or more. '>' excludes 100. '<=' and '==' do not match the requirement.

❓ security

advanced

Firestore Security Rules: Using == operator

You want to allow users to read their own profile document only if their user ID matches the document ID. Which security rule condition correctly enforces this?

Firebase

match /users/{userId} { allow read: if <condition>; }

A request.auth.uid == userId

B request.auth.uid < userId

C request.auth.uid != userId

D request.auth.uid >= userId

💡 Hint

Check for exact equality between authenticated user ID and document ID.

The '==' operator checks if the authenticated user's ID matches the document ID exactly, allowing access only to their own data. Other operators do not enforce this correctly.

✅ Best Practice

advanced

Choosing correct comparison operator in Firestore queries

You want to query Firestore for products priced less than or equal to $50. Which operator should you use in the where clause to get all products priced at $50 or less?

Firebase

firebase.firestore().collection('products').where('price', '<operator>', 50).get()

A '<'

B '<='

C '>'

D '=='

💡 Hint

Think about including the price 50 in the results.

The '<=' operator includes all products priced less than or equal to 50. '<' excludes 50. '>' and '==' do not match the requirement.

🧠 Conceptual

expert

Understanding Firebase Realtime Database query behavior with >= and == operators

Consider this Firebase Realtime Database query to find users with age greater than or equal to 21: firebase.database().ref('users').orderByChild('age').startAt(21).once('value') What will be the difference in results if you replace startAt(21) with equalTo(21)?

A Both return users aged 21 and older

B startAt(21) returns only users aged exactly 21; equalTo(21) returns users aged 21 and older

C startAt(21) returns users aged 21 and older; equalTo(21) returns only users aged exactly 21

D Both return only users aged exactly 21

💡 Hint

Consider how startAt and equalTo filter data differently.

startAt(21) returns all users with age 21 or more. equalTo(21) returns only users whose age is exactly 21.

Practice

1. Which comparison operator in Firebase queries checks if a field value is exactly equal to a given value?

easy

A. ==

B. <

C. >=

D. <=

Solution

1. Step 1: Understand the purpose of each operator

   The operator == checks for exact equality, while <, >=, and <= check for less than, greater or equal, and less or equal respectively.

2. Step 2: Identify the operator for exact equality

   Since the question asks for exact equality, the correct operator is ==.

3. Final Answer:

   == -> Option A

4. Quick Check:

   Exact equality operator = == [OK]

Hint: Use == to check if values are exactly the same [OK]

Common Mistakes:

• Confusing == with assignment operator =
• Using < or > for equality checks
• Thinking >= means equal only

2. Which of the following is the correct Firebase query syntax to find documents where the field age is greater than 18?

easy

A. query.where('age', '==', 18)

B. query.where('age', '<', 18)

C. query.where('age', '>=', 18)

D. query.where('age', '>', 18)

Solution

1. Step 1: Identify the operator for 'greater than'

   The operator for 'greater than' is >.

2. Step 2: Match the operator with the correct syntax

   The correct syntax uses query.where('age', '>', 18) to filter documents where age is greater than 18.

3. Final Answer:

   query.where('age', '>', 18) -> Option D

4. Quick Check:

   Greater than operator = > [OK]

Hint: Use '>' for values strictly greater than target [OK]

Common Mistakes:

• Using '==' instead of '>'
• Confusing '>=' with '>'
• Using '<' when wanting greater values

3. Given this Firebase query: query.where('score', '>=', 50).where('score', '<', 70), which scores will be included in the results?

medium

A. Scores greater than or equal to 50 and less than 70

B. Scores greater than 50 and less than or equal to 70

C. Scores equal to 50 or equal to 70

D. Scores equal to 50 and less than 70

Solution

1. Step 1: Analyze the first condition

   The first condition score >= 50 includes scores 50 and above.

2. Step 2: Analyze the second condition

   The second condition score < 70 includes scores less than 70, excluding 70 itself.

3. Step 3: Combine both conditions

   Only scores that satisfy both conditions are included: scores greater than or equal to 50 and less than 70.

4. Final Answer:

   Scores greater than or equal to 50 and less than 70 -> Option A

5. Quick Check:

   Range includes 50 ≤ score < 70 [OK]

Hint: Combine >= and < for range queries [OK]

Common Mistakes:

• Including 70 when using '<' operator
• Confusing '>' with '>='
• Thinking 'or' instead of 'and' between conditions

4. You wrote this Firebase query: query.where('price', '=>', 100). Why does it cause an error?

medium

A. Because 'price' field cannot be compared

B. Because 100 should be a string, not a number

C. Because '=>' is not a valid comparison operator in Firebase queries

D. Because the operator should be '<=' instead

Solution

1. Step 1: Check the operator syntax

   The operator '=>' is not a valid Firebase comparison operator. The correct operator for 'greater than or equal' is '>='.

2. Step 2: Understand Firebase operator rules

   Firebase only supports specific operators: ==, <, >, <=, and >=. Using any other operator causes a syntax error.

3. Final Answer:

   '=>' is invalid operator causing error -> Option C

4. Quick Check:

   Use >=, not => for greater or equal [OK]

Hint: Use >=, not => for greater or equal [OK]

Common Mistakes:

• Typing => instead of >=
• Using strings instead of numbers for numeric fields
• Assuming all operators are valid

5. You want to query Firebase documents where the field temperature is between 20 and 30 inclusive. Which query correctly uses comparison operators to achieve this?

hard

A. query.where('temperature', '>', 20).where('temperature', '<', 30)

B. query.where('temperature', '>=', 20).where('temperature', '<=', 30)

C. query.where('temperature', '==', 20).where('temperature', '==', 30)

D. query.where('temperature', '<=', 20).where('temperature', '>=', 30)

Solution

1. Step 1: Understand inclusive range requirements

   Inclusive means including both 20 and 30 values.

2. Step 2: Choose operators that include boundary values

   >= includes 20 and <= includes 30, so these operators match the requirement.

3. Step 3: Verify the combined query

   The query query.where('temperature', '>=', 20).where('temperature', '<=', 30) correctly filters temperatures from 20 to 30 inclusive.

4. Final Answer:

   query.where('temperature', '>=', 20).where('temperature', '<=', 30) -> Option B

5. Quick Check:

   Inclusive range uses >= and <= [OK]

Hint: Use >= and <= for inclusive ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using == twice filters only exact matches
• Reversing operators causing empty results