SQL database resolvers in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Resolving a simple SQL query in GraphQL

Given a GraphQL resolver that runs this SQL query:

SELECT id, name FROM users WHERE age > 30;

What will be the output of the resolver if the users table contains:

| id | name  | age |
|----|-------|-----|
| 1  | Alice | 25  |
| 2  | Bob   | 35  |
| 3  | Carol | 40  |

A []

B [{"id": 1, "name": "Alice"}]

C [{"id": 2, "name": "Bob"}]

D [{"id": 2, "name": "Bob"}, {"id": 3, "name": "Carol"}]

💡 Hint

Remember the WHERE clause filters users older than 30.

The query selects users with age greater than 30, so Bob (35) and Carol (40) are included.

🧠 Conceptual

intermediate

Understanding resolver batching in SQL database resolvers

Why is batching SQL queries in GraphQL resolvers beneficial?

A It increases the number of queries sent to the database.

B It reduces the number of database calls, improving performance.

C It makes the resolver code more complex without benefits.

D It prevents caching of query results.

💡 Hint

Think about how many times the database is accessed.

Batching combines multiple requests into one query, reducing database calls and speeding up response time.

📝 Syntax

advanced

Identify the syntax error in this SQL resolver code snippet

Which option contains the correct SQL query syntax used inside a GraphQL resolver?

SELECT * FROM orders WHERE user_id = $1;

A SELECT * FROM orders WHERE user_id = $1;

B SELECT * FROM orders WHERE user_id == $1;

C SELECT * FROM orders WHERE user_id := $1;

D SELECT * FROM orders WHERE user_id =? $1;

💡 Hint

Check the operator used for comparison in SQL.

SQL uses a single equals sign (=) for comparison, not double equals (==) or other operators.

❓ optimization

advanced

Optimizing SQL database resolvers for nested GraphQL queries

You have a GraphQL query requesting users and their posts. Which approach optimizes SQL database resolvers to avoid the N+1 query problem?

A Use a single SQL JOIN query to fetch users and their posts together.

B Fetch posts first, then fetch users separately.

C Fetch all users, then for each user fetch posts with separate queries.

D Fetch users and posts in two unrelated queries without joining.

💡 Hint

Think about how to reduce the number of queries to the database.

Using a JOIN fetches all needed data in one query, preventing multiple queries per user (N+1 problem).

🔧 Debug

expert

Debugging a resolver that returns empty results unexpectedly

A GraphQL resolver runs this SQL query:

SELECT * FROM products WHERE category = $1;

But it always returns an empty list even though the database has matching products. What is the most likely cause?

A The SQL query syntax is incorrect.

B The products table is empty.

C The variable $1 is not properly passed or bound in the resolver.

D The GraphQL schema does not define the products type.

💡 Hint

Check how variables are passed from GraphQL to SQL.

If the parameter $1 is not bound correctly, the query filters on NULL or empty value, returning no rows.