@Query for custom JPQL in Spring Boot - Mini Project: Build & Apply

Using @Query for Custom JPQL in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage a list of books in a library. You want to find books by their author's name using a custom JPQL query.

🎯 Goal: Create a Spring Data JPA repository with a custom JPQL query using the @Query annotation to find books by author name.

📋 What You'll Learn

Create an entity class Book with fields id, title, and author.

Create a repository interface BookRepository that extends JpaRepository.

Add a custom method findBooksByAuthorName in BookRepository using @Query with JPQL.

Use the custom query to find all books by a given author.

💡 Why This Matters

🌍 Real World

Custom JPQL queries let you write precise database queries in Spring Boot applications, useful when default query methods are not enough.

💼 Career

Understanding @Query with JPQL is essential for backend developers working with Spring Data JPA to efficiently retrieve data from databases.

1

Create the Book entity

Create a class called Book annotated with @Entity. Add private fields Long id annotated with @Id and @GeneratedValue, String title, and String author. Include public getters and setters.

Spring Boot

// Your code here to create the Book entity class with id, title, and author fields

Hint

Use @Entity on the class. Use @Id and @GeneratedValue on the id field.

2

Create the BookRepository interface

Create an interface called BookRepository that extends JpaRepository<Book, Long>. This interface will be used to access the database.

Spring Boot

import org.springframework.data.jpa.repository.JpaRepository;

// Your code here to create BookRepository interface extending JpaRepository

Hint

Use public interface BookRepository extends JpaRepository<Book, Long>.

3

Add custom JPQL query method

Inside BookRepository, add a method List<Book> findBooksByAuthorName(String authorName) annotated with @Query. Use JPQL: "SELECT b FROM Book b WHERE b.author = :authorName".

Spring Boot

import org.springframework.data.jpa.repository.Query;
import java.util.List;

// Add the method with @Query annotation here
// Your code here

Hint

Use @Query with JPQL string and @Param for the parameter.

4

Use the custom query method in a service

Create a class BookService with a private final BookRepository field. Add a constructor to inject BookRepository. Add a method List<Book> getBooksByAuthor(String authorName) that calls bookRepository.findBooksByAuthorName(authorName) and returns the result.

Spring Boot

import java.util.List;

// Your code here to create BookService class with constructor injection and getBooksByAuthor method

Hint

Use constructor injection for BookRepository and call the custom query method inside getBooksByAuthor.

Practice

1. What is the main purpose of using @Query annotation in Spring Data JPA?

easy

A. To write custom JPQL or SQL queries when default methods are insufficient

B. To automatically generate database tables

C. To configure database connection properties

D. To define entity relationships

Solution

1. Step 1: Understand default query methods

   Spring Data JPA provides default query methods like findById, but they are limited.

2. Step 2: Role of @Query

   @Query allows writing custom JPQL or SQL queries to handle complex or specific data retrieval needs.

3. Final Answer:

   To write custom JPQL or SQL queries when default methods are insufficient -> Option A

4. Quick Check:

   @Query purpose = custom queries [OK]

Hint: Remember @Query is for custom queries beyond defaults [OK]

Common Mistakes:

• Thinking @Query creates tables
• Confusing @Query with database config
• Assuming @Query defines entity relations

2. Which of the following is the correct syntax to define a custom JPQL query using @Query in a Spring Data JPA repository interface?

easy

A. @Query("SELECT u FROM User u WHERE u.name = :name") List<User> findByName(@Param("name") String name);

B. @Query(SELECT * FROM User WHERE name = :name) List<User> findByName(String name);

C. @Query("SELECT * FROM User WHERE name = ?1") List<User> findByName(String name);

D. @Query("FROM User WHERE name = ?") List<User> findByName(@Param("name") String name);

Solution

1. Step 1: Check JPQL syntax

   JPQL uses entity names and fields, not table names or * syntax.

2. Step 2: Parameter binding

   Named parameters use :paramName and must be linked with @Param("paramName") in method.

3. Final Answer:

   @Query("SELECT u FROM User u WHERE u.name = :name") List<User> findByName(@Param("name") String name); -> Option A

4. Quick Check:

   JPQL + named param + @Param = correct syntax [OK]

Hint: Use entity names and :param with @Param for correct JPQL [OK]

Common Mistakes:

• Using SQL syntax (*) instead of JPQL
• Missing @Param annotation for named parameters
• Using positional parameters incorrectly

3. Given the repository method:

@Query("SELECT u FROM User u WHERE u.age > :minAge")
List<User> findUsersOlderThan(@Param("minAge") int minAge);

What will be the result of calling findUsersOlderThan(30)?

medium

A. A list of User entities with age less than 30

B. A list of User entities with age equal to 30

C. A list of User entities with age greater than 30

D. A runtime error due to missing parameter

Solution

1. Step 1: Analyze the JPQL query

   The query selects users where age is greater than the parameter minAge.

2. Step 2: Understand method call

   Calling findUsersOlderThan(30) sets minAge to 30, so users older than 30 are returned.

3. Final Answer:

   A list of User entities with age greater than 30 -> Option C

4. Quick Check:

   minAge=30, query > minAge = users older than 30 [OK]

Hint: Check parameter value and query condition carefully [OK]

Common Mistakes:

• Confusing > with >= or =
• Assuming parameter is ignored
• Expecting users younger than 30

4. Identify the error in the following repository method:

@Query("SELECT u FROM User u WHERE u.email = :email")
List<User> findByEmail(String email);

medium

A. JPQL query uses wrong entity name

B. Missing @Param annotation for the email parameter

C. Return type should be User, not List<User>

D. Query should use native SQL syntax

Solution

1. Step 1: Check parameter binding

   The query uses a named parameter :email, so the method parameter must have @Param("email") annotation.

2. Step 2: Validate other parts

   Entity name User is correct, return type List<User> is valid, and JPQL syntax is correct.

3. Final Answer:

   Missing @Param annotation for the email parameter -> Option B

4. Quick Check:

   Named param requires @Param annotation [OK]

Hint: Always add @Param for named parameters in @Query [OK]

Common Mistakes:

• Omitting @Param causes runtime errors
• Confusing JPQL with native SQL
• Assuming return type must be single entity

5. You want to write a custom JPQL query using @Query to find all users whose name contains a given substring (case insensitive). Which of the following method definitions correctly achieves this?

hard

A. @Query("SELECT u FROM User u WHERE u.name LIKE :namePart") List<User> findByNameLike(@Param("namePart") String namePart);

B. @Query("SELECT u FROM User u WHERE u.name LIKE '%:namePart%'") List<User> findByNameContains(@Param("namePart") String namePart);

C. @Query("SELECT u FROM User u WHERE u.name = :namePart") List<User> findByNameExact(@Param("namePart") String namePart);

D. @Query("SELECT u FROM User u WHERE LOWER(u.name) LIKE LOWER(CONCAT('%', :namePart, '%'))") List<User> findByNameContainsIgnoreCase(@Param("namePart") String namePart);

Solution

1. Step 1: Understand case insensitive search

   Use LOWER() on both field and parameter to ignore case.

2. Step 2: Use LIKE with wildcards

   Concatenate '%' before and after parameter to find substring matches.

3. Step 3: Check parameter binding

   Named parameter :namePart is linked with @Param("namePart") correctly.

4. Final Answer:

   @Query("SELECT u FROM User u WHERE LOWER(u.name) LIKE LOWER(CONCAT('%', :namePart, '%'))") List<User> findByNameContainsIgnoreCase(@Param("namePart") String namePart); -> Option D

5. Quick Check:

   LOWER + LIKE + CONCAT + @Param = correct case-insensitive contains [OK]

Hint: Use LOWER() and CONCAT('%', param, '%') for case-insensitive contains [OK]

Common Mistakes:

• Using LIKE with parameter inside quotes disables binding
• Not using LOWER() for case insensitivity
• Using = instead of LIKE for substring search