JPA查询指定字段详解

一、JPA更新指定字段

JPA更新指定字段需要使用EntityManager的merge方法将实体对象与数据库表进行同步。通过给实体对象的属性赋值后进行merge操作将在数据库表中更新对应的字段值，如下示例代码所示：

public void updateFieldById(Long id, String fieldName, Object fieldValue) {
    T entity = entityManager.find(clazz, id);
    if (entity != null) {
        try {
            Field field = clazz.getDeclaredField(fieldName);
            field.setAccessible(true);
            field.set(entity, fieldValue);
            entityManager.merge(entity);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

二、JPA只查询指定字段

在JPA中，只需要指定实体类中需要查询的字段，就可以减少查询的数据量，提高查询效率。例如，使用@NamedQuery注解或者调用EntityManager的createNamedQuery方法来查询指定字段，如示例代码所示：

@NamedQuery(
        name = "findUserNameById", 
        query = "SELECT u.username FROM User u WHERE u.id = :id"
)
public class User {
    //...
}

public User findUserNameById(Long id) {
    TypedQuery<String> query = entityManager.createNamedQuery("findUserNameById", String.class);
    query.setParameter("id", id);
    String username = query.getSingleResult();
    return new User(username);
}

三、JPA查询返回指定字段

JPA查询返回指定字段需要自定义ResultClass，将查询返回的结果映射到自己定义的ResultClass中。自定义ResultClass需要使用@ConstructorResult注解，将查询返回的结果映射到实体类中的构造方法参数中，如下示例代码所示：

@SqlResultSetMapping(name = "UserInfoMapping", classes = {
        @ConstructorResult(targetClass = UserInfo.class, columns = {
            @ColumnResult(name = "username", type = String.class),
            @ColumnResult(name = "age", type = Integer.class),
            @ColumnResult(name = "address", type = String.class)
        })
})
public class User {
    //...
}

public List<UserInfo> getUsersInfo() {
    Query query = entityManager.createNativeQuery("SELECT username, age, address FROM users", "UserInfoMapping");
    return query.getResultList();
}

public class UserInfo {
    private String username;
    private Integer age;
    private String address;

    public UserInfo(String username, Integer age, String address) {
        this.username = username;
        this.age = age;
        this.address = address;
    }

    // getter and setter
}

四、JPA查询指定排序字段

使用JPA查询指定排序字段需要在查询语句中添加order by子句，并在order by后指定排序字段，如下示例代码所示：

public List<User> findAllUsersOrderByUsername() {
    return entityManager.createQuery("SELECT u FROM User u ORDER BY u.username", User.class).getResultList();
}

五、JPA查询某个字段

在JPA查询某个字段时，可以直接在查询语句中指定查询的字段，并使用getSingleResult方法获取结果集中的第一条记录，如下示例代码所示：

public String findUsernameById(Long id) {
    return entityManager.createQuery("SELECT u.username FROM User u WHERE u.id = :id", String.class)
        .setParameter("id", id)
        .getSingleResult();
}

六、JPA查询字段为null或为空

在JPA中查询字段为null或为空需要使用IS NULL或IS EMPTY，如下示例代码所示：

public List<User> findUsersWithNullAddress() {
    return entityManager.createQuery("SELECT u FROM User u WHERE u.address IS NULL OR u.address = ''", User.class)
        .getResultList();
}

七、JPA连表查询展示多表字段选取

在JPA中进行连表查询，可以使用JPQL语句实现。其中关联字段通过JPA关系映射进行关联，查询结果需要自定义ResultClass来映射多表字段，如下示例代码所示：

@Entity
public class User {
    @Id
    private Long id;
    private String username;
    private Integer age;
    @OneToOne(mappedBy = "user")
    private Address address;

    // getter and setter
}

@Entity
public class Address {
    @Id
    private Long id;
    private String province;
    private String city;
    private String area;
    @OneToOne
    private User user;

    // getter and setter
}

@SqlResultSetMapping(name = "UserAndAddressMapping", classes = {
        @ConstructorResult(targetClass = UserAndAddress.class, columns = {
            @ColumnResult(name = "username", type = String.class),
            @ColumnResult(name = "age", type = Integer.class),
            @ColumnResult(name = "province", type = String.class),
            @ColumnResult(name = "city", type = String.class),
            @ColumnResult(name = "area", type = String.class)
        })
})
public class UserAndAddress {
    private String username;
    private Integer age;
    private String province;
    private String city;
    private String area;

    public UserAndAddress(String username, Integer age, String province, String city, String area) {
        this.username = username;
        this.age = age;
        this.province = province;
        this.city = city;
        this.area = area;
    }

    // getter and setter
}

public List<UserAndAddress> findUserAndAddress() {
    Query query = entityManager.createQuery("SELECT new com.example.UserAndAddress(u.username, u.age, a.province, a.city, a.area) FROM User u JOIN u.address a", UserAndAddress.class);
    return query.getResultList();
}