使用 Spring Data JPA 选择一列

Select one column using Spring Data JPA

提问by Alax

Does anyone have any idea how to get a single column using Spring Data JPA? I created a repository like below in my Spring Boot project, but always get the {"cause":null,"message":"PersistentEntity must not be null!"}error when accessing the Restful URL.

有谁知道如何使用 Spring Data JPA 获取单个列？我在 Spring Boot 项目中创建了一个如下所示的存储库，但{"cause":null,"message":"PersistentEntity must not be null!"}在访问 Restful URL 时总是出现错误。

@RepositoryRestResource(collectionResourceRel = "users", path = "users")
public interface UsersRepository extends CrudRepository<Users, Integer> {

    @Query("SELECT u.userName  FROM Users u")
    public List<String> getUserName();
}

Then if I access the Restful URL like ../users/search/getUserName, I get the error: {"cause":null,"message":"PersistentEntity must not be null!"}

然后，如果我访问 Restful URL，例如../users/search/getUserName，我会收到错误消息： {"cause":null,"message":"PersistentEntity must not be null!"}

回答by Abhijeet Behare

This Works for me.

这对我有用。

public interface UserDataRepository extends JpaRepository<UserData, Long> {

    @Query(value = "SELECT emp_name FROM user_data", nativeQuery = true)
    public List<Object[]> findEmp_name();
}


System.out.println("data"+  userDataRepository.findEmp_name());

The above line gave me this result :

上面的行给了我这个结果：

data[abhijeet, abhijeet1, abhijeet2, abhijeet3, abhijeet4, abhijeet5]

数据[abhijeet, abhijeet1, abhijeet2, abhijeet3, abhijeet4, abhijeet5]

回答by Ethiraj

Concept is : In your entity class create a constructor with only required instant variables. And use that constructor in the repository method shown below.

概念是：在您的实体类中创建一个仅包含所需即时变量的构造函数。并在下面显示的存储库方法中使用该构造函数。

Lets say you have a interface Repository like below

假设您有一个如下所示的接口存储库

1. Repository implementation:

   public interface UserRepository<User> extends JpaRepository<User,String>
   {
       @Query(value = "select new com.org.User(usr.userId) from User usr where usr.name(:name))")
       List<User> findUserIdAlone(@Param("name") String user);
   }
   

2. In Controller

   @RestController
   public class UserController 
   {
       @Autowired
       private UserRepository<User> userRepository; 
   
       @Res
       public ResponseEntity<User> getUser(@PathVariable("usrname") String userName)
       {
           User resultUser = usrRepository.findUserIdAlone(userName);
           return ResponseEntity.ok(resultUser);
       }
   }
   
   public class User 
   {
   
       private String userId,userName;
   
       public User(String userId) 
       {
           this.userId=userId;
       }
       // setter and getters goes here
   }
   

1. 存储库实现：

   public interface UserRepository<User> extends JpaRepository<User,String>
   {
       @Query(value = "select new com.org.User(usr.userId) from User usr where usr.name(:name))")
       List<User> findUserIdAlone(@Param("name") String user);
   }
   

2. 在控制器中

   @RestController
   public class UserController 
   {
       @Autowired
       private UserRepository<User> userRepository; 
   
       @Res
       public ResponseEntity<User> getUser(@PathVariable("usrname") String userName)
       {
           User resultUser = usrRepository.findUserIdAlone(userName);
           return ResponseEntity.ok(resultUser);
       }
   }
   
   public class User 
   {
   
       private String userId,userName;
   
       public User(String userId) 
       {
           this.userId=userId;
       }
       // setter and getters goes here
   }
   

回答by Murder

If you need list all of the users, try select userName from Users, if you need one user use "where"look at spring data JPA http://docs.spring.io/spring-data/jpa/docs/current/reference/html/, try change CrudRepository to JpaRepository

如果您需要列出所有用户，请尝试select userName from Users，如果您需要一个用户使用"where"查看 spring 数据 JPA http://docs.spring.io/spring-data/jpa/docs/current/reference/html/，请尝试更改 CrudRepository到 JpaRepository

回答by Hatem Jaber

If you want to only return a single column you should look at Projections and Excerptswhich will allow you to filter specific columns and other things that are usefule.

如果您只想返回单个列，您应该查看Projections 和 Excerpts，这将允许您过滤特定的列和其他有用的内容。