Why @Query for custom JPQL in Spring Boot? - Purpose & Use Cases

The Big Idea

Discover how a simple annotation can save you hours of debugging messy database code!

The Scenario

Imagine you have a database with many tables and you want to get specific data by writing complex queries manually in your code.

You write SQL queries as strings everywhere, mixing them with your Java code.

The Problem

Writing raw SQL queries manually is error-prone and hard to maintain.

It mixes database logic with Java code, making your code messy and difficult to read.

Also, if the database changes, you have to find and update all those queries yourself.

The Solution

The @Query annotation lets you write custom JPQL queries directly in your repository interfaces.

This keeps your queries organized, readable, and close to your Java code without mixing raw SQL strings everywhere.

It also integrates well with Spring Data, so you get type safety and easier maintenance.

Before vs After

✗ Before

String sql = "SELECT * FROM users WHERE age > 30"; // raw SQL in code
List<User> users = entityManager.createNativeQuery(sql, User.class).getResultList();

✓ After

@Query("SELECT u FROM User u WHERE u.age > 30")
List<User> findUsersOlderThan30();

What It Enables

You can write clear, reusable, and maintainable database queries directly in your Java interfaces, making your data access layer clean and efficient.

Real Life Example

In an online store app, you want to find all orders placed in the last month with a total price above a certain amount.

Using @Query, you write this complex query once in your repository and call it easily from your service code.

Key Takeaways

Manual SQL queries scattered in code are hard to maintain.

@Query centralizes and organizes custom JPQL queries.

This improves readability, safety, and ease of updates.

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