BAEL-8040: Find distinct rows using Spring data JPA (eugenp#16798)

persistence-modules/spring-data-jpa-query-2/pom.xml

@@ -13,6 +13,18 @@
         <relativePath>../../parent-boot-3</relativePath>
     </parent>
 
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+                <configuration>
+                    <skip>true</skip>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
     <dependencies>
         <dependency>
             <groupId>org.springframework.boot</groupId>
@@ -74,11 +86,22 @@
             <artifactId>spring-security-test</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.hsqldb</groupId>
             <artifactId>hsqldb</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>${lombok.version}</version>
+            <scope>provided</scope>
+        </dependency>
     </dependencies>
 
     <properties>

persistence-modules/spring-data-jpa-query-2/src/main/java/com/baeldung/distinct/DistinctDataApplication.java

@@ -0,0 +1,13 @@
+package com.baeldung.distinct;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class DistinctDataApplication {
+
+    public static void main(String[] args) {
+        SpringApplication.run(DistinctDataApplication.class, args);
+    }
+
+}

persistence-modules/spring-data-jpa-query-2/src/main/java/com/baeldung/distinct/NameView.java

@@ -0,0 +1,7 @@
+package com.baeldung.distinct;
+
+public interface NameView {
+
+    String getName();
+
+}

persistence-modules/spring-data-jpa-query-2/src/main/java/com/baeldung/distinct/School.java

@@ -0,0 +1,34 @@
+package com.baeldung.distinct;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.OneToMany;
+import jakarta.persistence.Table;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.ToString;
+
+import java.util.List;
+
+@Entity
+@Table(name = "school")
+@Data
+@ToString(exclude = "students")
+@EqualsAndHashCode(of = "id")
+public class School {
+
+    @Id
+    @Column(name = "school_id")
+    @GeneratedValue(strategy= GenerationType.IDENTITY)
+    private int id;
+
+    @Column(name = "name", length = 100)
+    private String name;
+
+    @OneToMany(mappedBy = "school")
+    private List<Student> students;
+
+}

persistence-modules/spring-data-jpa-query-2/src/main/java/com/baeldung/distinct/SchoolRepository.java

@@ -0,0 +1,22 @@
+package com.baeldung.distinct;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+
+@Repository
+public interface SchoolRepository extends JpaRepository<School, Integer> {
+
+    List<School> findDistinctByStudentsBirthYear(int birthYear);
+
+    Long countDistinctByStudentsBirthYear(int birthYear);
+
+    @Query("SELECT DISTINCT sch.name FROM School sch JOIN sch.students stu WHERE stu.birthYear = :birthYear")
+    List<String> findDistinctSchoolNameByStudentsBirthYear(@Param("birthYear") int birthYear);
+
+    List<NameView> findDistinctNameByStudentsBirthYear(int birthYear);
+
+}

persistence-modules/spring-data-jpa-query-2/src/main/java/com/baeldung/distinct/Student.java

@@ -0,0 +1,34 @@
+package com.baeldung.distinct;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.ToString;
+
+@Entity
+@Table(name = "student")
+@Data
+@ToString(exclude = "school")
+@EqualsAndHashCode(of = "id")
+public class Student {
+
+    @Id
+    @Column(name = "student_id")
+    private int id;
+
+    @Column(name = "name", length = 100)
+    private String name;
+
+    @Column(name = "birth_year")
+    private int birthYear;
+
+    @ManyToOne
+    @JoinColumn(name = "school_id", referencedColumnName = "school_id")
+    private School school;
+
+}

persistence-modules/spring-data-jpa-query-2/src/test/java/com/baeldung/distinct/DistinctDataIntegrationTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.distinct;
+
+import jakarta.inject.Inject;
+
+import java.util.List;
+
+import org.springframework.boot.test.context.SpringBootTest;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+@SpringBootTest(classes = DistinctDataApplication.class, properties = {
+        "spring.jpa.show-sql=true",
+        "spring.jpa.properties.hibernate.format_sql=true",
+        "spring.jpa.generate-ddl=true",
+        "spring.jpa.defer-datasource-initialization=true",
+        "spring.sql.init.data-locations=classpath:school-data.sql"
+})
+class DistinctDataIntegrationTest {
+
+    @Inject
+    private SchoolRepository schoolRepository;
+
+    @Test
+    void whenFindDistinctByStudentsBirthYear_thenReturnOneSchoolEntity() {
+        List<School> schoolList = schoolRepository.findDistinctByStudentsBirthYear(2011);
+        assertThat(schoolList.size()).isEqualTo(1);
+        assertThat(schoolList.get(0).getId()).isEqualTo(2);
+    }
+
+    @Test
+    void whenCountDistinctByStudentsBirthYear_thenReturnOne() {
+        assertThat(schoolRepository.countDistinctByStudentsBirthYear(2011)).isEqualTo(1);
+    }
+
+    @Test
+    void whenFindDistinctSchoolNameByStudentsBirthYear_thenReturnOneSchoolName() {
+        List<String> schoolNameList = schoolRepository.findDistinctSchoolNameByStudentsBirthYear(2011);
+        assertThat(schoolNameList.size()).isEqualTo(1);
+        assertThat(schoolNameList.get(0)).isEqualTo("St Joesph's College");
+    }
+
+    @Test
+    void whenFindDistinctNameByStudentsBirthYear_thenReturnOneSchoolName() {
+        List<NameView> schoolNameList = schoolRepository.findDistinctNameByStudentsBirthYear(2011);
+        assertThat(schoolNameList.size()).isEqualTo(1);
+        assertThat(schoolNameList.get(0).getName()).isEqualTo("St Joesph's College");
+    }
+
+}

persistence-modules/spring-data-jpa-query-2/src/test/resources/school-data.sql

@@ -0,0 +1,14 @@
+INSERT INTO school(name) VALUES ('Queen''s College');
+INSERT INTO school(name) VALUES ('St Joesph''s College');
+INSERT INTO school(name) VALUES ('Maryknoll Convent School');
+
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058789, 'Chan Tai Man', 2010, 1);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058790, 'Wong Siu Ming', 2009, 1);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058791, 'Leung Kwok Wai', 2008, 1);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058792, 'Cheung Chi Kin', 2010, 1);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058793, 'Ho Man Keung', 2009, 1);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058794, 'Lee Ka Yiu', 2008, 2);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058795, 'Lau Ka Ho', 2010, 2);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058796, 'Ng Chi Ho', 2009, 2);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058797, 'Lam Hoi Chun', 2011, 2);
+INSERT INTO student (student_id, name, birth_year, school_id) VALUES (24058798, 'Yip Chun Hei', 2011, 2);