Query Auto Generation

Overview

Spring Data JPA can automatically generate database queries based solely on the method names you define in your repository interface.

• Based on specific naming conventions, a JPQL query is automatically created and executed. This feature is extremely useful for most CRUD operations and simple retrieval queries.
• But..집계를 하는데 join을 매우 많이 하면…그렇게 복잡한건 그냥 개발자가 논리적으로 하는게 더 나음
• Spring docs

• 2 ways
  • By method name
  • By @Query annotation

By Method Name

Rules

Keyword	Meaning
findBy	Find / Retrieve
countBy	Count
deleteBy	Delete
existsBy	Check for existence
TopN	Top N results
First	The first result

Example

Method Name	Example of Generated JPQL
findByUsername	SELECT m FROM Member m WHERE m.username = ?1
findByEmailAndAge	SELECT m FROM Member m WHERE m.email = ?1 AND m.age = ?2
findTop3ByOrderByAgeDesc	SELECT m FROM Member m ORDER BY m.age DESC LIMIT 3

List<Member> findByUsername(String username);
List<Member> findByAgeGreaterThan(int age);
List<Member> findByUsernameAndAge(String username, int age);
List<Member> findTop3ByOrderByAgeDesc();

• This approach is well-suited for simple searches, pagination, and sorting.
• If the query involves many complex conditions, you should consider using QueryDSL or @Query instead.

@Query annotation

For writing complex queries, you can use the @Query annotation to directly provide either a JPQL or a Native SQL query. This approach uses parameter binding (with @Param) to prevent SQL Injection and ensure type safety.

JPQL

@Query("SELECT m FROM Member m WHERE m.username = :username")
List<Member> findByUsername(@Param("username") String username);

Native Query

@Query(value = "SELECT * FROM member WHERE username = :username", nativeQuery = true)
List<Member> findByNativeUsername(@Param("username") String username);

JPQL vs. Native Query

Feature	JPQL (Java Persistence Query Language)	Native Query (Standard SQL)
Query Target	Entities and their fields	Database tables and columns
Database Dependency	✅ Database-independent	❌ Database-dependent
Best Use Case	General purpose, object-oriented queries	Complex queries, performance tuning, specific DB features
Portability	High (easy to switch databases)	Low (may require changes if the DB is switched
visual example:

Tip

• Use a Native Query when your query is highly complex or requires performance optimization.
• For most situations, JPQL is sufficient.
• Use @Query judiciously where needed to improve readability and maintainability.

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