JPA查詢指定字段詳解

一、JPA更新指定字段

JPA更新指定字段需要使用EntityManager的merge方法將實體對象與數據庫表進行同步。通過給實體對象的屬性賦值後進行merge操作將在數據庫表中更新對應的字段值，如下示例代碼所示：

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只查詢指定字段

在JPA中，只需要指定實體類中需要查詢的字段，就可以減少查詢的數據量，提高查詢效率。例如，使用@NamedQuery註解或者調用EntityManager的createNamedQuery方法來查詢指定字段，如示例代碼所示：

@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查詢返回指定字段

JPA查詢返回指定字段需要自定義ResultClass，將查詢返回的結果映射到自己定義的ResultClass中。自定義ResultClass需要使用@ConstructorResult註解，將查詢返回的結果映射到實體類中的構造方法參數中，如下示例代碼所示：

@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查詢指定排序字段

使用JPA查詢指定排序字段需要在查詢語句中添加order by子句，並在order by後指定排序字段，如下示例代碼所示：

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

五、JPA查詢某個字段

在JPA查詢某個字段時，可以直接在查詢語句中指定查詢的字段，並使用getSingleResult方法獲取結果集中的第一條記錄，如下示例代碼所示：

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查詢字段為null或為空

在JPA中查詢字段為null或為空需要使用IS NULL或IS EMPTY，如下示例代碼所示：

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連表查詢展示多表字段選取

在JPA中進行連表查詢，可以使用JPQL語句實現。其中關聯字段通過JPA關係映射進行關聯，查詢結果需要自定義ResultClass來映射多表字段，如下示例代碼所示：

@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();
}