1. Modelling many to many relations
https://www.baeldung.com/jpa-many-to-many   

2. Spring Boot with H2 Database
https://www.baeldung.com/spring-boot-h2-database

3. Accessing H2 Database runing within SpringBootApplication

How to enable H2 Database Server Mode in Spring Boot
https://stackoverflow.com/questions/55830010/how-to-enable-h2-database-server-mode-in-spring-boot

How to access in memory h2 database of one spring boot application from another spring boot application
https://stackoverflow.com/questions/43256295/how-to-access-in-memory-h2-database-of-one-spring-boot-application-from-another/43276769#43276769

4. Just to inform
### Not needed because of existing dependencies in pom.xml
spring.datasource.driverClassName=org.h2.Driver
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

5. Browsing H2 in mem database 

#### application.properties
spring.datasource.url=jdbc:h2:mem:testdb
spring.jpa.show-sql = true
spring.jpa.hibernate.ddl-auto=create
spring.datasource.username=sa
spring.datasource.password=password
spring.h2.console.enabled=true
spring.h2.console.path=/h2-console
spring.h2.console.settings.trace=true
spring.h2.console.settings.web-allow-others=true

#### pom.xml (needed to connect by h2-console)

        <dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

#### in source code of SpringBootApplication (needed to connect by dbeaver)

    @Bean(initMethod = "start", destroyMethod = "stop")
    public Server h2Server() throws SQLException {
        return Server.createTcpServer("-tcp", "-tcpAllowOthers", "-tcpPort", "9092");
    }


#### Settings for h2-console 
#### running on http://localhost:8080/h2-console/
Driver class: org.h2.Driver
JDBC URL: jdbc:h2:mem:testdb

##### Settings for dbeaver
URI template: jdbc:h2:tcp://localhost:9092/mem:testdb
Default database: mem.testdb


##### changes if database stored in a file
##### in application.properties:  
spring.datasource.url=jdbc:h2:./testdb
##### in dbeaver settings 
JDBC URL: jdbc:h2:tcp://localhost:9092/./testdb

6. Lifecycle
https://thorben-janssen.com/persist-save-merge-saveorupdate-whats-difference-one-use/

7. Why Set Is Better Than List in @ManyToMany
https://dzone.com/articles/why-set-is-better-than-list-in-manytomany