Querying with where clause in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Filtering documents with a single where clause

Given a Firestore collection users with documents containing fields age and city, what will be the output of this query?

db.collection('users').where('age', '>', 25).get()

Assuming the collection has these documents:
- {name: 'Alice', age: 30, city: 'NY'}
- {name: 'Bob', age: 22, city: 'LA'}
- {name: 'Carol', age: 27, city: 'NY'}
- {name: 'Dave', age: 24, city: 'SF'}

Firebase

db.collection('users').where('age', '>', 25).get()

A [{name: 'Alice', age: 30, city: 'NY'}, {name: 'Carol', age: 27, city: 'NY'}]

B [{name: 'Bob', age: 22, city: 'LA'}, {name: 'Dave', age: 24, city: 'SF'}]

C [{name: 'Alice', age: 30, city: 'NY'}, {name: 'Bob', age: 22, city: 'LA'}]

D []

💡 Hint

Think about which users have age greater than 25.

The query filters users where age is greater than 25. Alice (30) and Carol (27) meet this condition.

📝 Syntax

intermediate

Identify the syntax error in a Firestore where clause

Which option contains a syntax error in the Firestore query using where clause?

Firebase

db.collection('products').where('price' '>=', 100).get()

A db.collection('products').where('price', '>=', 100).get()

B db.collection('products').where('price', '>=', '100').get()

C db.collection('products').where('price', '=>', 100).get()

D db.collection('products').where('price' '>=', 100).get()

💡 Hint

Check the commas and operator spelling in the where clause.

Option D is missing a comma between 'price' and '>=' causing a syntax error.

❓ optimization

advanced

Optimizing multiple where clauses in Firestore

You want to query a collection orders to find documents where status is 'shipped' and total is greater than 50. Which query is optimized and valid in Firestore?

A db.collection('orders').where('status', '==', 'shipped' || 'total', '>', 50).get()

B db.collection('orders').where('status', '==', 'shipped').where('total', '>', 50).get()

C db.collection('orders').where('status' == 'shipped' && 'total' > 50).get()

D db.collection('orders').where('status', '==', 'shipped' && 'total', '>', 50).get()

💡 Hint

Firestore supports chaining multiple where clauses separately.

Option B chains two where clauses correctly. Other options have invalid syntax or unsupported operators.

🧠 Conceptual

advanced

Understanding Firestore where clause limitations

Which statement about Firestore where clause limitations is true?

A You cannot combine <code>where</code> clauses with <code>orderBy</code> on different fields without an index.

B You can use multiple inequality filters on different fields in a single query.

C Firestore allows <code>where</code> clauses with OR logic directly.

D You can use <code>where</code> clauses with array fields using the '!=' operator.

💡 Hint

Think about Firestore indexing and query constraints.

Firestore requires indexes for combining where and orderBy on different fields. Multiple inequality filters on different fields are not allowed. OR logic is not supported directly. '!=' operator is limited.

🔧 Debug

expert

Debugging a Firestore query with multiple where clauses

You run this query:
db.collection('employees').where('department', '==', 'sales').where('age', '<', 30).where('age', '>', 40).get()

What will happen when you run it?

A The query throws an error because multiple inequality filters on the same field with conflicting conditions are not allowed.

B The query returns employees in sales department with age between 30 and 40.

C The query returns employees in sales department with age less than 30 and greater than 40.

D The query returns all employees in sales department ignoring age filters.

💡 Hint

Check Firestore rules about multiple inequality filters on the same field.

Firestore allows multiple inequality filters on the same field. The conditions age < 30 AND age > 40 cannot both be true, so the query returns no documents.