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.

Practice

1. What is the main role of SQL database resolvers in a GraphQL server?

easy

A. To format the GraphQL query results into JSON

B. To define the GraphQL schema types and fields

C. To handle user authentication and authorization

D. To fetch or modify data in the SQL database based on GraphQL queries or mutations

Solution

1. Step 1: Understand the purpose of resolvers

   Resolvers are functions that connect GraphQL queries or mutations to the actual data source, here an SQL database.

2. Step 2: Identify the role in data fetching and modification

   Resolvers run SQL commands to get or change data as requested by the GraphQL operations.

3. Final Answer:

   To fetch or modify data in the SQL database based on GraphQL queries or mutations -> Option D

4. Quick Check:

   Resolvers connect queries to SQL data [OK]

Hint: Resolvers link GraphQL queries to SQL commands [OK]

Common Mistakes:

• Confusing resolvers with schema definitions
• Thinking resolvers handle authentication
• Assuming resolvers format output only

2. Which of the following is the correct way to define a resolver function for a GraphQL query named getUser that fetches user data from an SQL database?

easy

A. const resolvers = { Query: { getUser: () => db.query('SELECT * FROM users') } };

B. const resolvers = { Mutation: { getUser: () => db.query('SELECT * FROM users') } };

C. const resolvers = { Query: { getUser: 'SELECT * FROM users' } };

D. const resolvers = { getUser: () => db.query('SELECT * FROM users') };

Solution

1. Step 1: Identify the correct resolver structure for a query

   Resolvers for queries are defined inside the Query object with function values.

2. Step 2: Check the syntax for calling the database query

   The resolver should be a function returning the result of db.query with the SQL command string.

3. Final Answer:

   const resolvers = { Query: { getUser: () => db.query('SELECT * FROM users') } }; -> Option A

4. Quick Check:

   Query resolvers are functions inside Query object [OK]

Hint: Query resolvers go inside Query object as functions [OK]

Common Mistakes:

• Placing query resolver inside Mutation
• Assigning SQL string directly instead of a function
• Defining resolver outside Query or Mutation objects

3. Given the resolver below, what will be the output when the GraphQL query { getUserById(id: 2) { name } } is executed?

const resolvers = {
  Query: {
    getUserById: (_, args) => {
      const users = [
        { id: 1, name: 'Alice' },
        { id: 2, name: 'Bob' },
        { id: 3, name: 'Carol' }
      ];
      return users.find(user => user.id === args.id);
    }
  }
};

medium

A. { "data": { "getUserById": { "name": "Alice" } } }

B. { "data": { "getUserById": { "name": "Bob" } } }

C. { "data": { "getUserById": null } }

D. Error: args.id is undefined

Solution

1. Step 1: Understand the resolver logic

   The resolver searches the users array for an object with id equal to args.id.

2. Step 2: Match the query argument with the array data

   The query requests id: 2, so the resolver returns the user with id: 2, which is { id: 2, name: 'Bob' }.

3. Final Answer:

   { "data": { "getUserById": { "name": "Bob" } } } -> Option B

4. Quick Check:

   Find user with id 2 = Bob [OK]

Hint: Find user by id in array using args.id [OK]

Common Mistakes:

• Confusing id 2 with id 1 or 3
• Forgetting to access args.id
• Assuming resolver returns entire user object without filtering

4. Identify the error in this resolver code snippet for a mutation that inserts a new user into the SQL database:

const resolvers = {
  Mutation: {
    addUser: (parent, args) => {
      db.query(`INSERT INTO users (name, email) VALUES (${args.name}, ${args.email})`);
      return { success: true };
    }
  }
};

medium

A. SQL injection risk due to string interpolation without parameterization

B. Missing return statement for the inserted user data

C. Incorrect resolver placement inside Mutation instead of Query

D. Using backticks instead of single quotes for SQL string

Solution

1. Step 1: Analyze the SQL query construction

   The SQL command uses string interpolation to insert args.name and args.email directly into the query string.

2. Step 2: Identify security risk

   This approach allows SQL injection attacks if user input contains malicious SQL code. Proper parameterization or prepared statements are needed.

3. Final Answer:

   SQL injection risk due to string interpolation without parameterization -> Option A

4. Quick Check:

   Unsafe string interpolation causes SQL injection [OK]

Hint: Never insert variables directly into SQL strings [OK]

Common Mistakes:

• Ignoring SQL injection vulnerability
• Thinking backticks cause errors
• Expecting mutation in Query object

5. You want to write a resolver that fetches all orders for a user, but only if the user is active. The SQL tables are users(id, active) and orders(id, user_id, total). Which resolver implementation correctly enforces this condition efficiently?

hard

A. const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM orders WHERE user_id = ${args.userId}`) } };

B. const resolvers = { Query: { userOrders: (_, args) => { const user = db.query(`SELECT * FROM users WHERE id = ${args.userId}`); if (user.active) { return db.query(`SELECT * FROM orders WHERE user_id = ${args.userId}`); } else { return []; } } } };

C. const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM orders WHERE user_id = ${args.userId} AND EXISTS (SELECT 1 FROM users WHERE id = ${args.userId} AND active = TRUE)`) } };

D. const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM users WHERE id = ${args.userId} AND active = TRUE`) } };

Solution

1. Step 1: Understand the requirement

   Fetch orders only if the user is active, combining the check and data fetch efficiently.

2. Step 2: Evaluate each option

   const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM orders WHERE user_id = ${args.userId} AND EXISTS (SELECT 1 FROM users WHERE id = ${args.userId} AND active = TRUE)`) } }; uses a single SQL query with EXISTS to check user active status and fetch orders in one call, which is efficient and correct.
   const resolvers = { Query: { userOrders: (_, args) => { const user = db.query(`SELECT * FROM users WHERE id = ${args.userId}`); if (user.active) { return db.query(`SELECT * FROM orders WHERE user_id = ${args.userId}`); } else { return []; } } } }; fetches user first then orders separately, causing multiple queries and possible async issues.
   const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM orders WHERE user_id = ${args.userId}`) } }; fetches orders without checking user active status.
   const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM users WHERE id = ${args.userId} AND active = TRUE`) } }; fetches user data only, not orders.

3. Final Answer:

   const resolvers = { Query: { userOrders: (_, args) => db.query(`SELECT * FROM orders WHERE user_id = ${args.userId} AND EXISTS (SELECT 1 FROM users WHERE id = ${args.userId} AND active = TRUE)`) } }; -> Option C

4. Quick Check:

   Single query with EXISTS enforces condition efficiently [OK]

Hint: Use EXISTS in SQL to combine condition and fetch in one query [OK]

Common Mistakes:

• Running multiple queries instead of one
• Not checking user active status before fetching orders
• Returning user data instead of orders