Querying with where clause in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to query documents where the field 'age' equals 25.

Firebase

const query = collectionRef.where('age', '==', [1]);

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

A35

B25

C20

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong number in the filter.

Confusing the operator '==' with other operators.

The where clause filters documents where the 'age' field is exactly 25.

2fill in blank

medium

Complete the code to query documents where the field 'status' is 'active'.

Firebase

const activeUsers = collectionRef.where('status', '==', [1]);

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

A'inactive'

B'pending'

C'active'

D'deleted'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the string value.

Using the wrong status value.

The query filters documents where the 'status' field is exactly 'active'.

3fill in blank

hard

Fix the error in the query to find documents where 'score' is greater than 80.

Firebase

const highScores = collectionRef.where('score', [1], 80);

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

A==

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>'.

Using '<' which finds lower scores.

The operator '>' means greater than, so it finds documents with score above 80.

4fill in blank

hard

Fill both blanks to query documents where 'age' is less than 30 and 'status' is 'active'.

Firebase

const query = collectionRef.where('age', [1], 30).where('status', [2], 'active');

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

A<

B==

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' for age.

Using '!=' instead of '==' for status.

The first condition filters ages less than 30, the second filters status equal to 'active'.

5fill in blank

hard

Fill all three blanks to query documents where 'score' is greater than 50, 'age' is less than 40, and 'status' is 'active'.

Firebase

const complexQuery = collectionRef.where('score', [1], 50).where('age', [2], 40).where('status', [3], 'active');

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

A>

B<

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up comparison operators.

Using '!=' instead of '==' for status.

The query filters documents with score > 50, age < 40, and status equal to 'active'.

Practice

1. What does the where clause do in a Firebase query?

easy

A. Filters documents based on a condition

B. Deletes documents from a collection

C. Adds new documents to a collection

D. Sorts documents alphabetically

Solution

1. Step 1: Understand the purpose of where clause

   The where clause is used to filter documents in a collection based on a condition.

2. Step 2: Compare with other options

   Deleting, adding, or sorting documents are different operations and not done by where.

3. Final Answer:

   Filters documents based on a condition -> Option A

4. Quick Check:

   where clause = filter condition [OK]

Hint: Remember: where filters, not adds or deletes [OK]

Common Mistakes:

• Confusing filtering with deleting data
• Thinking where adds new documents
• Assuming where sorts data

2. Which of the following is the correct syntax to query documents where the field age is equal to 25 in Firebase?

easy

A. db.collection('users').filter('age', '==', 25)

B. db.collection('users').where('age' = 25)

C. db.collection('users').where('age', '=', 25)

D. db.collection('users').where('age', '==', 25)

Solution

1. Step 1: Recall Firebase where clause syntax

   The correct syntax uses where(field, operator, value) with operator as a string, e.g., '=='.

2. Step 2: Check each option

   db.collection('users').where('age', '==', 25) matches correct syntax. db.collection('users').where('age' = 25) uses '=' instead of '=='. db.collection('users').filter('age', '==', 25) uses 'filter' which is incorrect. db.collection('users').where('age', '=', 25) uses '=' instead of '=='.

3. Final Answer:

   db.collection('users').where('age', '==', 25) -> Option D

4. Quick Check:

   where(field, '==', value) syntax [OK]

Hint: Use '==' operator as a string in where clause [OK]

Common Mistakes:

• Using '=' instead of '==' in where clause
• Using filter() instead of where()
• Missing quotes around operator

3. Given the following Firebase query:

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

Which products will be returned?

medium

A. Products with price less than 100

B. Products with price equal to 100 only

C. Products with price greater than or equal to 100

D. All products regardless of price

Solution

1. Step 1: Understand the operator '>=' in where clause

   The operator '>=' means greater than or equal to the given value.

2. Step 2: Apply the condition to the query

   The query returns products where price is 100 or more.

3. Final Answer:

   Products with price greater than or equal to 100 -> Option C

4. Quick Check:

   price >= 100 means price ≥ 100 [OK]

Hint: '>=' means greater or equal, so include 100 and above [OK]

Common Mistakes:

• Thinking '>=' means only greater than
• Confusing '<' with '>='
• Assuming it returns all products

4. What is wrong with this Firebase query?

db.collection('orders').where('status', 'equals', 'shipped').get()

medium

A. The operator 'equals' is invalid in where clause

B. The collection name 'orders' is incorrect

C. The field 'status' cannot be queried

D. The get() method is missing parentheses

Solution

1. Step 1: Check the operator used in where clause

   Firebase supports operators like '==', '<', '>=', but not 'equals'.

2. Step 2: Verify other parts of the query

   Collection name and field are valid, and get() is correctly called with parentheses.

3. Final Answer:

   The operator 'equals' is invalid in where clause -> Option A

4. Quick Check:

   Use '==' not 'equals' in where clause [OK]

Hint: Use '==' operator, not words like 'equals' [OK]

Common Mistakes:

• Using 'equals' instead of '=='
• Missing parentheses on get()
• Assuming any string operator works

5. You want to get all users from the 'users' collection whose age is between 18 and 30 inclusive. Which Firebase query correctly achieves this?

hard

A. db.collection('users').where('age', '>', 18).where('age', '<', 30)

B. db.collection('users').where('age', '>=', 18).where('age', '<=', 30)

C. db.collection('users').where('age', '==', 18).where('age', '==', 30)

D. db.collection('users').where('age', '>=', 18 || '<=', 30)

Solution

1. Step 1: Understand how to filter a range in Firebase

   Firebase allows chaining multiple where clauses to filter a range, e.g., age >= 18 and age <= 30.

2. Step 2: Analyze each option

   db.collection('users').where('age', '>=', 18).where('age', '<=', 30) correctly uses two where clauses with '>=' and '<=' operators. db.collection('users').where('age', '>', 18).where('age', '<', 30) excludes 18 and 30 by using '>' and '<'. db.collection('users').where('age', '==', 18).where('age', '==', 30) tries to match age exactly 18 and 30 simultaneously, which is impossible. db.collection('users').where('age', '>=', 18 || '<=', 30) uses invalid syntax combining operators in one where.

3. Final Answer:

   db.collection('users').where('age', '>=', 18).where('age', '<=', 30) -> Option B

4. Quick Check:

   Chain where with >= and <= for inclusive range [OK]

Hint: Chain where clauses for range with >= and <= [OK]

Common Mistakes:

• Using '==' twice for range
• Combining operators incorrectly in one where
• Using > and < excludes boundary values