Spring Data JPA 多个实体类表联合视图查询

Spring Data JPA 查询数据库时，若是两个表有关联，那么就设个外键，在查询的时候用Specification建立Join 查询即可。可是只支持左链接，不支持右链接，虽然说左右链接反过来就能实现同样的效果，可是这就关系到谁是谁的外键的问题。外键搞起来有时候确实麻烦。因此为了查询个视图，没有找到更好的办法，只好在service层查两次合并起来了。

    @Entity
    @Table(name="tb_user")
    public class UserInfo implements Serializable{
     
        @Id
        @GeneratedValue(strategy=GenerationType.IDENTITY)
        private Long userId;
        private String userName;
        private String password;
        private String name;
        private int age;
        private String sex;
        private String email;
        private Date dateOfBirth;
        private String telNumber;
        private String education;
        private String school;
    //    @ManyToOne
    //    @JoinColumn(name="addressId")
    //    private Address address;
        private Long addressId;
    // getter and setter
    }

    @Entity
    @Table(name="tb_address")
    public class Address implements Serializable{
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long addressId;　　 private Long userId 
        private String areaCode;
        private String country;
        private String province;
        private String city;
        private String area;
        private String detailAddress;
    // getter and setter
    }

建立一个类包含UserInfo和Address中的全部属性：

    public class ViewInfo implements Serializable{
     
        private UserInfo userInfo;
        private Address address;
        public ViewInfo(){
     
        }
        public ViewInfo(UserInfo userInfo){
            Address address = new Address();
            this.userInfo = userInfo;
            this.address = address;
        }
        public ViewInfo(Address address){
            UserInfo userInfo = new UserInfo();
            this.userInfo = userInfo;
            this.address = address;
        }
        public ViewInfo(UserInfo userInfo,Address address){
            this.userInfo = userInfo;
            this.address = address;
        }
    // getter and setter
    }

接下来就是在DAO层中写自定义查询语句了：

    public interface UserInfoRepository extends CrudRepository<UserInfo, Long>{
        @Query(value="SELECT new com.demo.test.Entity.ViewInfo(u,a)FROM "
        + " UserInfo u, com.demo.test.Entity.Address a WHERE u.addressId = a.id) ")
        List<ViewInfo> findViewInfo();
        @Query("SELECT new com.demo.test.Entity.ViewInfo"
        + "(u) FROM UserInfo u WHERE u.addressId IS NULL OR u.addressId NOT IN (SELECT a.id FROM Address a)")
        List<ViewInfo> findViewInfoLeft();
        @Query("SELECT new com.demo.test.Entity.ViewInfo"
        + "(a) FROM Address a WHERE a.id NOT IN (SELECT u.addressId FROM UserInfo u WHERE u.addressId IS NOT NULL)")
        List<ViewInfo> findViewInfoRight();
     
    }

而后在service层中查询各个部分：

        public void summary(){
          System.out.println("=======middle part=======");
          List<ViewInfo> userInfos = userInfoRepository.findViewInfo();
          for(ViewInfo item : userInfos){
              System.out.println(item.getUserInfo().getUserName()+" "+item.getAddress().getCity());
          }
          System.out.println("=======left part=======");
          List<ViewInfo> userInfoLeft = userInfoRepository.findViewInfoLeft();
          for(ViewInfo item : userInfoLeft){
              System.out.println(item.getUserInfo().getUserName()+" "+item.getAddress().getCity());
          }
          System.out.println("=======right part=======");
          List<ViewInfo> userInfoRight = addressRepository.findViewInfoRight();
          for(ViewInfo item : userInfoRight){
              System.out.println(item.getUserInfo().getUserName()+" "+item.getAddress().getCity());
          }
     
        }

数据库的Inner Join选交集，Outer Join 选并集，Left Join 选左表与右表的差集加上交集，Right Join选右表与左表的差集加上交集。暂且如此了。若是哪位看到个人文章有更好的方法请不吝赐教。