Query builder in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS QueryBuilder code?

Consider this code snippet using TypeORM's QueryBuilder in a NestJS service. What will be the output of the query execution?

NestJS

const users = await this.userRepository.createQueryBuilder('user')
  .where('user.age > :age', { age: 18 })
  .andWhere('user.isActive = :active', { active: true })
  .getMany();

return users.length;

A Number of all users regardless of age or active status

B Number of users older than 18 and active

C Number of users older than 18 regardless of active status

D Number of users active regardless of age

💡 Hint

Look at the where and andWhere conditions carefully.

The query filters users with age > 18 and isActive true, so the result length is the count of users matching both conditions.

📝 Syntax

intermediate

Which option correctly adds a LEFT JOIN in NestJS QueryBuilder?

You want to join the 'profile' relation of 'user' entity using a LEFT JOIN. Which code snippet is correct?

A createQueryBuilder('user').joinAndSelect('user.profile', 'profile')

B createQueryBuilder('user').join('user.profile', 'profile')

C createQueryBuilder('user').leftJoin('profile', 'user.profile')

D createQueryBuilder('user').leftJoinAndSelect('user.profile', 'profile')

💡 Hint

LEFT JOIN with selection requires a specific method.

leftJoinAndSelect performs a LEFT JOIN on the relation and selects its fields. join() is an INNER JOIN by default, and joinAndSelect() performs an INNER JOIN with selection.

🔧 Debug

advanced

Why does this QueryBuilder code throw an error?

This code throws a runtime error. What is the cause?

NestJS

const result = await this.userRepository.createQueryBuilder('user')
  .where('user.name = :name')
  .setParameters({ name: 'Alice' })
  .getOne();

A No error; code runs and returns user named Alice

B setParameters is called after where without parameters, causing error

C Missing parameter value in where clause causes error

D Using setParameters instead of passing parameters in where causes error

💡 Hint

Check how parameters are passed and when setParameters is called.

Calling where with a parameter placeholder and then setParameters before getOne is valid and works correctly.

❓ state_output

advanced

What is the value of 'count' after running this QueryBuilder code?

Given this code, what will be the value of 'count'?

NestJS

const count = await this.userRepository.createQueryBuilder('user')
  .where('user.isActive = :active', { active: true })
  .getCount();

A Total number of all users regardless of active status

B Number of users returned by getMany()

C Total number of active users in the database

D Number of users with isActive false

💡 Hint

getCount() returns the count of records matching the query.

getCount() returns the number of users where isActive is true, matching the where condition.

🧠 Conceptual

expert

Which option best describes how QueryBuilder handles SQL injection prevention?

How does NestJS QueryBuilder (TypeORM) protect against SQL injection when using parameters?

A It automatically escapes parameters passed via placeholders like :param

B It requires manual escaping of all parameters before passing

C It disables parameter binding by default for performance

D It does not protect; raw SQL must be sanitized manually

💡 Hint

Think about how parameters are passed safely in QueryBuilder.

QueryBuilder uses parameter binding with placeholders and automatically escapes values to prevent SQL injection.