Extras telusko

- for configuring h2 database(in memory database)
  - make sure package is there in pom.xml
  - add following in `application.properties`
    ```yml
	spring.h2.console.enabled=true
	spring.datasource.platform=h2
	spring.datasource.url=jdbc:h2:mem:telusko
	```
	here telusko is name of database
  - now start your application and go to `localhost:8080/h2~console` and u will get a login page. now click on test connection button
- for configuring JPA
  - make sure respective dependency is there and at `application.properties` along with spring.datasource properties one more is mentioned for say mysql 
`spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect`
  - in POJO add annotation `Entity` above class defintion and in instance say `aid` add `@Id` annotation
  - now a table with fields of those POJO instance name will be created
  - u can add `data.sql` in `src/main/resources` to add initial data
  - in order to do crud operations use CrudRepository which is a part of spring data commons which is part of JPA
  - Create this interface
   ```java
	package com.example.demo;

	import org.springframework.data.repository.CrudRepository;

	public interface AlienRepo extends CrudRepository<Other,Integer> {


	} 

	```
	and then in your main file/controller to save data use
	```java
	public class HomeController(){
		@Autowire
		Alienrepo repo;

		 @RequestMapping("/addAlien")
		public String addAlien(Alien alien){ 
			repo.save(alien);
			return "home";
		}
		
		@RequestMapping("/getAlien")
		public ModelAndView getAlien(@RequestParam int aid){ 
			ModelAndView mv= new ModelAndView("showAlien");
			Alien alien=repo.findById(aid).orElse(new Alien());
			mv.addObject(alien);
			return mv;
		}
	}
	```
- More query methods for interface Repo
  - In the following code, these other query methods are defined by just declaring then by following certain conventions like `findBy<PropertyName><Operation>` here property like Tech,Aid(first letter must be capital and other as small)
  - everything is not configured with conventions so  for complex queries `Query` annotation have to be used. `?1` is used instead of `?`  bcz there can be other placeholders too and its to index them.
  ```java
	 package com.example.demo;

	 import org.springframework.data.repository.CrudRepository;

	 public interface AlienRepo extends CrudRepository<Other,Integer> {

		List<Alien> findByTech(String tech);
		 
		 List<Alien> findByAidGreaterThan(int aid);
		 
		 // jpql instead of sql
		 @Query("from Alien where tech=?1 order by aname")
		 List<Alien> findByTechSorted(String tech);
	 } 
	```