Spring Data JPA 多个实体类表联合视图查询

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Spring Data JPA 查询数据库时，如果两个表有关联，那么就设个外键，在查询的时候用Specification创建Join 查询便可。但是只支持左连接，不支持右连接，虽说左右连接反过来就能实现一样的效果，但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。所以为了查询个视图，没有找到更好的办法，只好在service层查两次合并起来了。

两个实体类：

@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
}

创建一个类包含UserInfo和Address中的所有属性：

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();}

然后在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。如果哪位看到我的文章有更好的方法请不吝赐教。