Query builder in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Query Builder in NestJS?

A Query Builder in NestJS is a tool that helps you create database queries step-by-step using code instead of writing raw SQL. It makes queries easier to build, read, and maintain.

beginner

How do you start a Query Builder for a User entity in NestJS using TypeORM?

You use the repository for the User entity and call createQueryBuilder(). For example: const query = this.userRepository.createQueryBuilder('user');

beginner

What method do you use to add a WHERE condition in a NestJS Query Builder?

You use the .where() method. For example: query.where('user.age > :age', { age: 18 }) adds a condition to filter users older than 18.

intermediate

How can you join related tables using Query Builder in NestJS?

You use .leftJoinAndSelect() or .innerJoinAndSelect() methods to join related tables and select their data. For example: query.leftJoinAndSelect('user.profile', 'profile') joins the profile table.

beginner

Why is using a Query Builder better than raw SQL in NestJS?

Query Builder helps avoid SQL syntax errors, improves security by preventing SQL injection, and makes queries easier to read and maintain with code instead of raw strings.

Which method starts a Query Builder for an entity in NestJS?

A createQueryBuilder()

B find()

C execute()

D connect()

createQueryBuilder() is the method used to start building a query for an entity.

How do you add a WHERE clause in a NestJS Query Builder?

A .orderBy()

B .join()

C .select()

D .where()

The .where() method adds conditions to filter results.

Which method joins related tables and selects their data?

A .leftJoinAndSelect()

B .groupBy()

C .limit()

D .insert()

.leftJoinAndSelect() joins tables and includes their data in the result.

What is a key benefit of using Query Builder over raw SQL?

A Requires no database connection

B Prevents SQL injection

C Runs faster than SQL

D Automatically creates tables

Query Builder helps prevent SQL injection by safely handling parameters.

In NestJS, which object do you usually call createQueryBuilder() on?

A Service

B Controller

C Repository

D Module

You call createQueryBuilder() on the repository of the entity.

Explain how to build a query with a WHERE condition and a JOIN using NestJS Query Builder.

Think about chaining methods to build the query step-by-step.

Describe the advantages of using Query Builder in NestJS compared to writing raw SQL queries.

Focus on safety, readability, and flexibility.

Practice

1. What is the main purpose of using a QueryBuilder in NestJS with TypeORM?

easy

A. To build database queries safely and readably without raw SQL

B. To write raw SQL queries directly in the code

C. To create frontend UI components

D. To manage user authentication

Solution

1. Step 1: Understand QueryBuilder role

   QueryBuilder helps create database queries in a safe and readable way, avoiding raw SQL.

2. Step 2: Identify incorrect options

   Options B, C, and D do not describe QueryBuilder's purpose correctly.

3. Final Answer:

   To build database queries safely and readably without raw SQL -> Option A

4. Quick Check:

   QueryBuilder purpose = safe, readable queries [OK]

Hint: QueryBuilder avoids raw SQL for safer queries [OK]

Common Mistakes:

• Confusing QueryBuilder with raw SQL usage
• Thinking QueryBuilder manages UI or auth
• Assuming QueryBuilder is for frontend

2. Which of the following is the correct way to start a QueryBuilder for an entity named User in NestJS with TypeORM?

easy

A. this.userRepository.startQuery('user')

B. this.userRepository.queryBuilder('user')

C. this.userRepository.buildQuery('user')

D. this.userRepository.createQueryBuilder('user')

Solution

1. Step 1: Recall QueryBuilder method

   The correct method to start a QueryBuilder is createQueryBuilder.

2. Step 2: Match method with options

   Only this.userRepository.createQueryBuilder('user') uses createQueryBuilder correctly.

3. Final Answer:

   this.userRepository.createQueryBuilder('user') -> Option D

4. Quick Check:

   Start QueryBuilder = createQueryBuilder() [OK]

Hint: Use createQueryBuilder() to start queries [OK]

Common Mistakes:

• Using incorrect method names like queryBuilder or buildQuery
• Forgetting to call createQueryBuilder on repository
• Passing wrong arguments to createQueryBuilder

3. Given the following code snippet, what will be the output of users?

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

medium

A. A single user with age exactly 18

B. An array of users with age greater than 18

C. An array of all users regardless of age

D. An error because of wrong parameter syntax

Solution

1. Step 1: Analyze the where condition

   The query filters users where age > 18, so only users older than 18 are selected.

2. Step 2: Understand getMany() result

   getMany() returns an array of matching users, not a single user or error.

3. Final Answer:

   An array of users with age greater than 18 -> Option B

4. Quick Check:

   Where age > 18 + getMany() = filtered array [OK]

Hint: getMany() returns all matching records as array [OK]

Common Mistakes:

• Confusing getMany() with getOne()
• Misreading the where condition syntax
• Expecting all users without filter

4. Identify the error in this QueryBuilder code snippet:

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

medium

A. The parameter name in where clause does not match the object key

B. Missing await keyword before createQueryBuilder

C. Using getOne() instead of getMany() causes error

D. The alias 'user' is not allowed in QueryBuilder

Solution

1. Step 1: Check parameter binding

   The where clause uses :name but the object key is username, causing mismatch.

2. Step 2: Understand effect of mismatch

   This mismatch means the parameter :name is undefined, causing query error or no results.

3. Final Answer:

   The parameter name in where clause does not match the object key -> Option A

4. Quick Check:

   Parameter keys must match placeholders [OK]

Hint: Match parameter keys exactly with placeholders [OK]

Common Mistakes:

• Using different names for parameters and placeholders
• Forgetting to await async calls
• Misunderstanding getOne() usage

5. You want to fetch users who are either older than 30 or have the role 'admin'. Which QueryBuilder code correctly implements this condition?

hard

A. this.userRepository.createQueryBuilder('user')
.where('user.age > 30 OR user.role = admin')
.getMany();

B. this.userRepository.createQueryBuilder('user')
.where('user.age > :age AND user.role = :role', { age: 30, role: 'admin' })
.getMany();

C. this.userRepository.createQueryBuilder('user')
.where('user.age > :age', { age: 30 })
.orWhere('user.role = :role', { role: 'admin' })
.getMany();

D. this.userRepository.createQueryBuilder('user')
.where('user.age > :age OR user.role = :role', { age: 30 })
.getMany();

Solution

1. Step 1: Understand the OR condition

   We want users older than 30 OR with role 'admin', so we use where and orWhere.

2. Step 2: Check parameter usage

   this.userRepository.createQueryBuilder('user') .where('user.age > :age', { age: 30 }) .orWhere('user.role = :role', { role: 'admin' }) .getMany(); correctly uses where with age and orWhere with role, both with parameters.

3. Step 3: Identify errors in other options

   this.userRepository.createQueryBuilder('user') .where('user.age > :age AND user.role = :role', { age: 30, role: 'admin' }) .getMany(); uses AND instead of OR. this.userRepository.createQueryBuilder('user') .where('user.age > 30 OR user.role = admin') .getMany(); lacks parameter binding. this.userRepository.createQueryBuilder('user') .where('user.age > :age OR user.role = :role', { age: 30 }) .getMany(); misses role parameter.

4. Final Answer:

   this.userRepository.createQueryBuilder('user') .where('user.age > :age', { age: 30 }) .orWhere('user.role = :role', { role: 'admin' }) .getMany(); -> Option C

5. Quick Check:

   OR condition = where + orWhere with params [OK]

Hint: Use where() then orWhere() for OR conditions [OK]

Common Mistakes:

• Using AND instead of OR in where clause
• Not binding parameters properly
• Combining conditions in one where without orWhere