Why compound queries narrow results in Firebase - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Firebase

db.collection('users').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

Choosing a status that does not match the intended filter.

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

2fill in blank

medium

Complete the code to add a second condition to the query to filter users with age greater than 18.

Firebase

db.collection('users').where('status', '==', 'active').where('age', [1], 18)

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

A<=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' or '<' which select younger users.

The query filters users older than 18 by using the '>' operator.

3fill in blank

hard

Fix the error in the compound query that tries to filter by 'status' and 'age' but uses an invalid operator.

Firebase

db.collection('users').where('status', '==', 'active').where('age', [1], 18)

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

A>

Barray-contains

C!=

Din

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'array-contains' or 'in' with numeric fields.

The '>' operator is valid for numeric comparisons like age. 'array-contains' and 'in' are for arrays or lists, not numbers.

4fill in blank

hard

Fill both blanks to create a compound query filtering users with 'status' 'active' and 'score' greater than 50.

Firebase

db.collection('users').where('status', [1], [2]).where('score', '>', 50)

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

A'=='

B'active'

C'inactive'

D'pending'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operator or status value.

The first condition checks if 'status' equals 'active'.

5fill in blank

hard

Fill all three blanks to create a compound query filtering users with 'status' 'active', 'score' greater than 50, and 'age' less than 30.

Firebase

db.collection('users').where('status', [1], [2]).where('score', '>', 50).where('age', [3], 30)

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

A'=='

B'active'

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators for age or status.

The query filters users with status 'active', score greater than 50, and age less than 30.

Practice

1. What is the main reason compound queries in Firebase narrow down results?

easy

A. They increase the number of documents returned.

B. They combine multiple conditions to filter data more precisely.

C. They ignore some conditions to speed up queries.

D. They randomly select documents from the database.

Solution

1. Step 1: Understand what compound queries do

   Compound queries use more than one condition to filter data.

2. Step 2: Effect of combining conditions

   Combining conditions means only documents matching all conditions are returned, narrowing results.

3. Final Answer:

   They combine multiple conditions to filter data more precisely. -> Option B

4. Quick Check:

   Compound queries = multiple conditions = narrower results [OK]

Hint: Think: More conditions mean fewer matching results [OK]

Common Mistakes:

• Thinking compound queries increase results
• Believing conditions are ignored
• Assuming random selection happens

2. Which of the following is the correct syntax to create a compound query in Firebase Firestore?

easy

A. db.collection('users').query('age', '>=', 18).query('active', '==', true)

B. db.collection('users').filter('age', '>=', 18).filter('active', '==', true)

C. db.collection('users').select('age', 'active')

D. db.collection('users').where('age', '>=', 18).where('active', '==', true)

Solution

1. Step 1: Recall Firestore query syntax

   Firestore uses chained where methods to add conditions.

2. Step 2: Identify correct chaining

   db.collection('users').where('age', '>=', 18).where('active', '==', true) correctly chains two where calls for compound conditions.

3. Final Answer:

   db.collection('users').where('age', '>=', 18).where('active', '==', true) -> Option D

4. Quick Check:

   Compound query syntax uses chained where() calls [OK]

Hint: Remember: Firestore uses chained where() for multiple conditions [OK]

Common Mistakes:

• Using filter() instead of where()
• Using query() which is invalid
• Using select() which only picks fields

3. Given this Firestore query:
db.collection('products').where('price', '<=', 100).where('inStock', '==', true)
What kind of results will this query return?

medium

A. Products priced at 100 or less and currently in stock

B. All products priced above 100 and in stock

C. Products priced at 100 or less regardless of stock

D. Only products that are out of stock

Solution

1. Step 1: Analyze the first condition

   The query filters products with price less than or equal to 100.

2. Step 2: Analyze the second condition

   It also filters products where inStock is true, meaning available products.

3. Final Answer:

   Products priced at 100 or less and currently in stock -> Option A

4. Quick Check:

   Compound query narrows to price ≤ 100 AND inStock = true [OK]

Hint: Both conditions must be true for results [OK]

Common Mistakes:

• Ignoring the inStock condition
• Thinking price > 100 is included
• Confusing AND with OR logic

4. You wrote this query:
db.collection('orders').where('status', '==', 'shipped').where('status', '==', 'delivered')
Why does this query return no results?

medium

A. Because the collection 'orders' does not exist.

B. Because the syntax for multiple where clauses is incorrect.

C. Because a document cannot have 'status' equal to both 'shipped' and 'delivered' at the same time.

D. Because Firestore does not support queries on the 'status' field.

Solution

1. Step 1: Understand the conditions on the same field

   The query asks for documents where 'status' equals 'shipped' AND 'delivered' simultaneously.

2. Step 2: Realize the logical impossibility

   A single field cannot have two different values at once, so no documents match both.

3. Final Answer:

   Because a document cannot have 'status' equal to both 'shipped' and 'delivered' at the same time. -> Option C

4. Quick Check:

   Same field with conflicting conditions returns no results [OK]

Hint: Same field can't equal two different values simultaneously [OK]

Common Mistakes:

• Thinking syntax is wrong
• Assuming Firestore disallows querying 'status'
• Assuming collection absence causes this

5. You want to find users who are both 'active' and have a 'score' greater than 50. Which compound query correctly narrows results to this criteria?

hard

A. db.collection('users').where('active', '==', true).where('score', '>', 50)

B. db.collection('users').where('active', '==', true).where('score', '<', 50)

C. db.collection('users').where('active', '==', false).where('score', '>', 50)

D. db.collection('users').where('active', '==', true).where('score', '==', 50)

Solution

1. Step 1: Identify the conditions needed

   Users must be 'active' (true) and have 'score' greater than 50.

2. Step 2: Match conditions to query options

   db.collection('users').where('active', '==', true).where('score', '>', 50) matches both conditions correctly: active equals true and score greater than 50.

3. Final Answer:

   db.collection('users').where('active', '==', true).where('score', '>', 50) -> Option A

4. Quick Check:

   Correct compound query matches all criteria [OK]

Hint: Match conditions exactly to narrow results [OK]

Common Mistakes:

• Using wrong comparison operators
• Mixing true/false for active
• Using equality instead of greater than