# Database with TypeORM (Node.js / NestJS)

> Entities, repositories, relations, migrations, query builder, transactions, cascades

- 30 interview questions
- Mid-Level
- [Interview Questions: Node.js / NestJS](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions.md)

## 1. What is an Entity in TypeORM?

**Answer**

An Entity in TypeORM is a TypeScript class decorated with @Entity() that represents a database table. Each instance of the Entity corresponds to a row in the table. Class properties are automatically mapped to table columns using decorators like @Column().

## 2. Which decorator to use to mark a property as a column in TypeORM?

**Answer**

The @Column() decorator transforms a class property into a database column. TypeORM automatically infers the SQL type from the TypeScript type, but it can be explicitly specified with @Column('varchar', { length: 255 }). This decorator is essential for all columns except the primary key.

## 3. What is the difference between @PrimaryColumn() and @PrimaryGeneratedColumn()?

**Answer**

@PrimaryGeneratedColumn() creates a primary key auto-incremented by the database, ideal for numeric identifiers. @PrimaryColumn() defines a primary key whose value must be provided manually, useful for UUID identifiers or composite keys. For most cases, using @PrimaryGeneratedColumn() is simpler and safer.

## 27 more questions available

- How to inject a TypeORM repository into a NestJS service?
- Which decorator to use to define a nullable column in TypeORM?

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