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。
@Entity
@Table(name="tb_user")
publicclassUserInfoimplements Serializable{
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Long userId;
private String userName;
private String password;
private String name;
privateint 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")
publicclassAddressimplements Serializable{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
privateLong 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：
publicclassViewInfoimplements 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：
publicinterfaceUserInfoRepositoryextendsCrudRepository{
@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 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 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 findViewInfoRight();
}
作者：一位不著名的普通群众
链接：https://www.jianshu.com/p/31aa998b86ee
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