# Repositories and Derived Queries — Spring Boot

Source: https://www.skillbyai.com/en/spring-boot/j-repo

> Interfaces, no implementation.

## JpaRepository and method-name queries

Extending `JpaRepository<Product, Long>` gives CRUD, paging and sorting methods without writing an implementation. **Derived queries** turn method names into queries: `findByPriceLessThanOrderByPriceAsc(max)` filters and sorts. For complex queries use `@Query` with JPQL or native SQL, or projections that select only needed columns. Watch for the **N+1 query problem** when loading related entities lazily in loops; use fetch joins or entity graphs.

## A repository with a derived query

This file is from a demo "shop" service generated by start.spring.io for Spring Boot 4.1.1 and Java 21 (Temurin 21.0.12), built with Maven 3.9.16; the project compiled and its tests passed. Spring Data generates the implementation and the query from the method name.

```java
package com.example.shop;

import java.math.BigDecimal;
import java.util.List;
import org.springframework.data.jpa.repository.JpaRepository;

public interface ProductRepository extends JpaRepository<Product, Long> {
    // Spring Data derives the query from the method name
    List<Product> findByPriceLessThanOrderByPriceAsc(BigDecimal max);
}
```

## Log SQL while developing

Temporarily enable SQL logging to see the queries Hibernate generates and catch N+1 problems early.

**Quiz:** How does Spring Data implement findByPriceLessThan?

- [ ] You must write the SQL in a separate class
- [x] It derives the query from the method name
- [ ] It loads all rows and filters in Java always
- [ ] It calls an external API

*Answer:* It derives the query from the method name. Naming conventions become queries.