@Entity
@Table(name = "customers")
@NoArgsConstructor
@AllArgsConstructor
@Data
public class Customer {
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Integer id;
  @Column(name = "name", unique = true, nullable = false, length = 255)
  private String name;
  @Column(name = "region")
  @Enumerated(EnumType.STRING)
  private Region region;
  public enum Region {
@Repository
public interface CustomerRepository extends JpaRepository<Customer, Integer> {}




    

I created a simple service to insert a new entity in it
@Service
public class CustomerService {
  private final CustomerRepository customerRepository;
  public CustomerService(CustomerRepository customerRepository) {
    this.customerRepository = customerRepository;
  @Transactional
  public Customer createCustomer(String name, Customer.Region region) {
    Customer customer = new Customer();
    customer.setName(name);
    customer.setRegion(region);
    return customerRepository.save(customer);
I created this test (run using in memory H2 database):
@SpringBootTest
@Transactional
class CustomerServiceTest {
  @Autowired
  private CustomerService customerService;
  @Test
  public void test_createCustomer() {
    Customer createdCustomer = customerService.createCustomer("The Crusher", Customer.Region.EU);
    Assertions.assertEquals(4, createdCustomer.getId(), "Id should match");
    Assertions.assertEquals("The Crusher", createdCustomer.getName(), "Name should match");
    Assertions.assertEquals(Customer.Region.EU, createdCustomer.getRegion(), "Region should match");
with this data.sql fixture:
INSERT INTO customers (id, name, region) VALUES (1, 'Dr. Carmack', 'EU'), (2, 'Pinky', 'EU'), (3, 'Revenant', 'EU');
I explicitly set spring.jpa.defer-datasource-initialization: true in my test application.yml
When I run this test, I've got an exception, and I can't figure why:
org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint ["PRIMARY KEY ON PUBLIC.CUSTOMERS(ID) ( /* key:1 */ 1, 'Dr. Carmack', 'EU')"; SQL statement:
insert into customers (id, name, region) values (default, ?, ?) [23505-214]]
There is a sample project here: https://github.com/fistons/issue-spring-data-test
          
This issue tracker is for bug reports and feature/improvement requests.

This question is more of a usage questions. Those should be asked at Stackoverflow and be tagged with spring-data-jpa or in this case actually jpa.

By using Stackoverflow, the community can assist and the questions and their answers can more easily be found using the search engine of your choice.
But since you are here:

You insert data using data.sql with fixed id values. When you next insert data with JPA the identity column will create the same ids and you'll get a Unique Key Violation.
To fix this use either ids that won't get generated by the id column, e.g. negative values.

Alternatively don't fix the ids in data.sql and let the database generate them.
I can't be sure because you didn't included the stack trace.
          
Thanks for the answer!
I'll indeed post the question on stackoverflow because there is something I don't get. Even with fixed ids in the data.sql, H2  should increments the id by it self, as the id column is declared @GeneratedValue(strategy=IDENTITY).
In the logs, we can also see that hibernate generate this statement id integer generated by default as identity
Shouldn't the next generated id be 4 in this case?
Here is the full logs
[INFO] 
[INFO] --------------------------< com.example:demo >--------------------------
[INFO] Building demo 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-resources-plugin:3.3.0:resources (default-resources) @ demo ---
[INFO] Copying 1 resource
[INFO] Copying 0 resource
[INFO] 
[INFO] --- maven-compiler-plugin:3.10.1:compile (default-compile) @ demo ---
[INFO] Nothing to compile - all classes are up to date
[INFO] 
[INFO] --- maven-resources-plugin:3.3.0:testResources (default-testResources) @ demo ---
[INFO] Copying 2 resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.10.1:testCompile (default-testCompile) @ demo ---
[INFO] Nothing to compile - all classes are up to date
[INFO] 
[INFO] --- maven-surefire-plugin:2.22.2:test (default-test) @ demo ---
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.example.demo.CustomerServiceTest
09:14:37.718 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Neither @ContextConfiguration nor @ContextHierarchy found for test class [CustomerServiceTest]: using SpringBootContextLoader
09:14:37.721 [main] DEBUG org.springframework.test.context.support.AbstractContextLoader - Could not detect default resource locations for test class [com.example.demo.CustomerServiceTest]: no resource found for suffixes {-context.xml, Context.groovy}.
09:14:37.722 [main] INFO org.springframework.test.context.support.AnnotationConfigContextLoaderUtils - Could not detect default configuration classes for test class [com.example.demo.CustomerServiceTest]: CustomerServiceTest does not declare any static, non-private, non-final, nested classes annotated with @Configuration.
09:14:37.733 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Using ContextCustomizers for test class [CustomerServiceTest]: [ExcludeFilterContextCustomizer, DuplicateJsonObjectContextCustomizer, MockitoContextCustomizer, TestRestTemplateContextCustomizer, DisableObservabilityContextCustomizer, PropertyMappingContextCustomizer, Customizer]
09:14:37.768 [main] DEBUG org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider - Identified candidate component class: file [/home/emercier/dev/DIP/issues/demo/target/classes/com/example/demo/DemoApplication.class]
09:14:37.769 [main] INFO org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Found @SpringBootConfiguration com.example.demo.DemoApplication for test class com.example.demo.CustomerServiceTest
09:14:37.808 [main] DEBUG org.springframework.test.context.util.TestContextSpringFactoriesUtils - Skipping candidate TestExecutionListener [org.springframework.test.context.web.ServletTestExecutionListener] due to a missing dependency. Specify custom TestExecutionListener classes or make the default TestExecutionListener classes and their required dependencies available. Offending class: [jakarta/servlet/ServletContext]
09:14:37.818 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Using TestExecutionListeners for test class [CustomerServiceTest]: [DirtiesContextBeforeModesTestExecutionListener, ApplicationEventsTestExecutionListener, MockitoTestExecutionListener, DependencyInjectionTestExecutionListener, DirtiesContextTestExecutionListener, TransactionalTestExecutionListener, SqlScriptsTestExecutionListener, EventPublishingTestExecutionListener, WithSecurityContextTestExecutionListener, ReactorContextTestExecutionListener, ResetMocksTestExecutionListener, RestDocsTestExecutionListener, MockRestServiceServerResetTestExecutionListener, MockMvcPrintOnlyOnFailureTestExecutionListener, WebDriverTestExecutionListener, MockWebServiceServerTestExecutionListener]
09:14:37.819 [main] DEBUG org.springframework.test.context.support.AbstractDirtiesContextTestExecutionListener - Before test class: class [CustomerServiceTest], class annotated with @DirtiesContext [false] with mode [null]
09:14:37.833 [main] DEBUG org.springframework.test.context.support.DependencyInjectionTestExecutionListener - Performing dependency injection for test class com.example.demo.CustomerServiceTest
2023-02-10T09:14:38.024+01:00  INFO 16706 --- [           main] com.example.demo.CustomerServiceTest     : Starting CustomerServiceTest using Java 17.0.2 with PID 16706 (started by emercier in /home/emercier/dev/DIP/issues/demo)
2023-02-10T09:14:38.025+01:00  INFO 16706 --- [           main] com.example.demo.CustomerServiceTest     : No active profile set, falling back to 1 default profile: "default"
2023-02-10T09:14:38.225+01:00  INFO 16706 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data JPA repositories in DEFAULT mode.
2023-02-10T09:14:38.257+01:00  INFO 16706 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 28 ms. Found 1 JPA repository interfaces.
2023-02-10T09:14:38.520+01:00  INFO 16706 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Starting...
2023-02-10T09:14:38.676+01:00  INFO 16706 --- [           main] com.zaxxer.hikari.pool.HikariPool        : HikariPool-1 - Added connection conn0: url=jdbc:h2:mem:a712f157-ed3a-4ce1-8ad3-193f2f6416a9 user=SA
2023-02-10T09:14:38.677+01:00  INFO 16706 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Start completed.
2023-02-10T09:14:38.717+01:00  INFO 16706 --- [           main] o.hibernate.jpa.internal.util.LogHelper  : HHH000204: Processing PersistenceUnitInfo [name: default]
2023-02-10T09:14:38.753+01:00  INFO 16706 --- [           main] org.hibernate.Version                    : HHH000412: Hibernate ORM core version 6.1.6.Final
2023-02-10T09:14:38.867+01:00  WARN 16706 --- [           main] org.hibernate.orm.deprecation            : HHH90000021: Encountered deprecated setting [javax.persistence.sharedCache.mode], use [jakarta.persistence.sharedCache.mode] instead
2023-02-10T09:14:38.972+01:00  INFO 16706 --- [           main] SQL dialect                              : HHH000400: Using dialect: org.hibernate.dialect.H2Dialect
2023-02-10T09:14:39.435+01:00 DEBUG 16706 --- [           main] org.hibernate.SQL                        : 
    drop table if exists customers cascade 
Hibernate: 
    drop table if exists customers cascade 
2023-02-10T09:14:39.443+01:00 DEBUG 16706 --- [           main] org.hibernate.SQL                        : 
    create table customers (
       id integer generated by default as identity,
        name varchar(255) not null,
        region varchar(255),
        primary key (id)
Hibernate: 
    create table customers (
       id integer generated by default as identity,
        name varchar(255) not null,
        region varchar(255),
        primary key (id)
2023-02-10T09:14:39.451+01:00 DEBUG 16706 --- [           main] org.hibernate.SQL                        : 
    alter table if exists customers 
       add constraint UK_to73biqfrti0j9f7k12l255w5 unique (name)
Hibernate: 
    alter table if exists customers 
       add constraint UK_to73biqfrti0j9f7k12l255w5 unique (name)
2023-02-10T09:14:39.454+01:00  INFO 16706 --- [           main] o.h.e.t.j.p.i.JtaPlatformInitiator       : HHH000490: Using JtaPlatform implementation: [org.hibernate.engine.transaction.jta.platform.internal.NoJtaPlatform]
2023-02-10T09:14:39.462+01:00  INFO 16706 --- [           main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit 'default'
2023-02-10T09:14:39.807+01:00  INFO 16706 --- [           main] com.example.demo.CustomerServiceTest     : Started CustomerServiceTest in 1.956 seconds (process running for 2.46)
2023-02-10T09:14:39.955+01:00 DEBUG 16706 --- [           main] org.hibernate.SQL                        : 
    insert 
        customers
        (id, name, region) 
    values
        (default, ?, ?)
Hibernate: 
    insert 
        customers
        (id, name, region) 
    values
        (default, ?, ?)
2023-02-10T09:14:39.963+01:00  WARN 16706 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 23505, SQLState: 23505
2023-02-10T09:14:39.964+01:00 ERROR 16706 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : Unique index or primary key violation: "PRIMARY KEY ON PUBLIC.CUSTOMERS(ID) ( /* key:1 */ 1, 'Dr. Carmack', 'EU')"; SQL statement:
insert into customers (id, name, region) values (default, ?, ?) [23505-214]
[ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 2.369 s <<< FAILURE! - in com.example.demo.CustomerServiceTest
[ERROR] test_createCustomer  Time elapsed: 0.189 s  <<< ERROR!
org.springframework.dao.DataIntegrityViolationException: 
could not execute statement; SQL [n/a]; constraint ["PRIMARY KEY ON PUBLIC.CUSTOMERS(ID) ( /* key:1 */ 1, 'Dr. Carmack', 'EU')"; SQL statement:
insert into customers (id, name, region) values (default, ?, ?) [23505-214]]
        at org.springframework.orm.jpa.vendor.HibernateJpaDialect.convertHibernateAccessException(HibernateJpaDialect.java:270)
        at org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:233)
        at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.translateExceptionIfPossible(AbstractEntityManagerFactoryBean.java:550)
        at org.springframework.dao.support.ChainedPersistenceExceptionTranslator.translateExceptionIfPossible(ChainedPersistenceExceptionTranslator.java:61)
        at org.springframework.dao.support.DataAccessUtils.translateIfNecessary(DataAccessUtils.java:242)
        at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:152)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:163)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:218)
        at jdk.proxy2/jdk.proxy2.$Proxy98.save(Unknown Source)
        at com.example.demo.CustomerService.createCustomer(CustomerService.java:25)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:343)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:196)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
        at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:752)
        at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123)
        at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388)
        at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:752)
        at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:703)
        at com.example.demo.CustomerService$$SpringCGLIB$$0.createCustomer(<generated>)
        at com.example.demo.CustomerServiceTest.test_createCustomer(CustomerServiceTest.java:20)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:727)
        at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
        at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
        at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
        at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)
        at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:86)
        at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(InterceptingExecutableInvoker.java:103)
        at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.lambda$invoke$0(InterceptingExecutableInvoker.java:93)
        at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
        at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
        at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
        at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
        at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:92)
        at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:86)
        at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$7(TestMethodTestDescriptor.java:217)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:213)
        at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:138)
        at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:68)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:151)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
        at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
        at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
        at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
        at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
        at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
        at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
        at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
        at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:35)
        at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
        at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:54)
        at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:220)
        at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:188)
        at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:202)
        at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:181)
        at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
        at org.apache.maven.surefire.junitplatform.JUnitPlatformProvider.invokeAllTests(JUnitPlatformProvider.java:150)
        at org.apache.maven.surefire.junitplatform.JUnitPlatformProvider.invoke(JUnitPlatformProvider.java:124)
        at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:384)
        at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:345)
        at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:126)
        at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:418)
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
        at org.hibernate.exception.internal.SQLExceptionTypeDelegate.convert(SQLExceptionTypeDelegate.java:60)
        at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:56)
        at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:109)
        at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:95)
        at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:200)
        at org.hibernate.dialect.identity.GetGeneratedKeysDelegate.executeAndExtract(GetGeneratedKeysDelegate.java:57)
        at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:42)
        at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3327)
        at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3965)
        at org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:80)
        at org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:653)
        at org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:283)
        at org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:264)
        at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:322)
        at org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:340)
        at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:286)
        at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:192)
        at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:122)
        at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:184)
        at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:129)
        at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:53)
        at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:107)
        at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:737)
        at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:721)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:307)
        at jdk.proxy2/jdk.proxy2.$Proxy94.persist(Unknown Source)
        at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:610)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at org.springframework.data.repository.core.support.RepositoryMethodInvoker$RepositoryFragmentMethodInvoker.lambda$new$0(RepositoryMethodInvoker.java:288)
        at org.springframework.data.repository.core.support.RepositoryMethodInvoker.doInvoke(RepositoryMethodInvoker.java:136)
        at org.springframework.data.repository.core.support.RepositoryMethodInvoker.invoke(RepositoryMethodInvoker.java:120)
        at org.springframework.data.repository.core.support.RepositoryComposition$RepositoryFragments.invoke(RepositoryComposition.java:516)
        at org.springframework.data.repository.core.support.RepositoryComposition.invoke(RepositoryComposition.java:285)
        at org.springframework.data.repository.core.support.RepositoryFactorySupport$ImplementationMethodExecutionInterceptor.invoke(RepositoryFactorySupport.java:628)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.doInvoke(QueryExecutorMethodInterceptor.java:168)
        at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.invoke(QueryExecutorMethodInterceptor.java:143)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:77)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123)
        at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388)
        at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
        at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137)
        ... 89 more
Caused by: org.h2.jdbc.JdbcSQLIntegrityConstraintViolationException: Unique index or primary key violation: "PRIMARY KEY ON PUBLIC.CUSTOMERS(ID) ( /* key:1 */ 1, 'Dr. Carmack', 'EU')"; SQL statement:
insert into customers (id, name, region) values (default, ?, ?) [23505-214]
        at org.h2.message.DbException.getJdbcSQLException(DbException.java:508)
        at org.h2.message.DbException.getJdbcSQLException(DbException.java:477)
        at org.h2.message.DbException.get(DbException.java:223)
        at org.h2.message.DbException.get(DbException.java:199)
        at org.h2.mvstore.db.MVPrimaryIndex.add(MVPrimaryIndex.java:120)
        at org.h2.mvstore.db.MVTable.addRow(MVTable.java:519)
        at org.h2.command.dml.Insert.insertRows(Insert.java:174)
        at org.h2.command.dml.Insert.update(Insert.java:135)
        at org.h2.command.CommandContainer.executeUpdateWithGeneratedKeys(CommandContainer.java:242)
        at org.h2.command.CommandContainer.update(CommandContainer.java:163)
        at org.h2.command.Command.executeUpdate(Command.java:252)
        at org.h2.jdbc.JdbcPreparedStatement.executeUpdateInternal(JdbcPreparedStatement.java:209)
        at org.h2.jdbc.JdbcPreparedStatement.executeUpdate(JdbcPreparedStatement.java:169)
        at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeUpdate(ProxyPreparedStatement.java:61)
        at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeUpdate(HikariProxyPreparedStatement.java)
        at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:197)
        ... 136 more
2023-02-10T09:14:40.064+01:00  INFO 16706 --- [ionShutdownHook] j.LocalContainerEntityManagerFactoryBean : Closing JPA EntityManagerFactory for persistence unit 'default'
2023-02-10T09:14:40.065+01:00  INFO 16706 --- [ionShutdownHook] .SchemaDropperImpl$DelayedDropActionImpl : HHH000477: Starting delayed evictData of schema as part of SessionFactory shut-down'
2023-02-10T09:14:40.065+01:00 DEBUG 16706 --- [ionShutdownHook] org.hibernate.SQL                        : 
    drop table if exists customers cascade 
Hibernate: 
    drop table if exists customers cascade 
2023-02-10T09:14:40.083+01:00  INFO 16706 --- [ionShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown initiated...
2023-02-10T09:14:40.088+01:00  INFO 16706 --- [ionShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown completed.
[INFO] 
[INFO] Results:
[INFO] 
[ERROR] Errors: 
[ERROR]   CustomerServiceTest.test_createCustomer:20 » DataIntegrityViolation could not ...
[INFO] 
[ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.524 s
[INFO] Finished at: 2023-02-10T09:14:40+01:00
[INFO] ------------------------------------------------------------------------
Again, I will post this question on stackoverflow, I justed wanted to add some context.
Thanks again!
I don't think so.

The ids are not generated based on the ids in the table but a separate construct.
Feel free to leave a link to the SO question here.
          
Yes, here it is: https://stackoverflow.com/questions/75409593/dataintegrityviolationexception-when-trying-to-insert-an-entity-in-a-h2-table.
What bother me the most is that I have another project with the same kind of tests, and it works. I'm trying to reproduce it on my spare time and keep this ticket updated