java代码实现mybatis-plus连表分页查询

假设我们有两个实体类`User`和`Order`，它们之间是一对多的关系，即一个用户可以有多个订单。我们需要实现一个连表分页查询的功能，可以按照用户的某个字段进行排序。

1. 首先在`pom.xml`文件中添加Mybatis-Plus和MySql的依赖：

<!-- Mybatis-Plus依赖 -->
<dependency>
    <groupId>com.baomidou</groupId>
    <artifactId>mybatis-plus-boot-starter</artifactId>
    <version>3.4.0</version>
</dependency>

<!-- MySql驱动依赖 -->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.25</version>
</dependency>

2. 创建两个实体类`User`和`Order`，并在`User`类中添加`@TableId`注解：

@Data
public class User {
    @TableId(type = IdType.AUTO)
    private Long id;
    private String name;
    private Integer age;
    private String email;
}

@Data
public class Order {
    private Long id;
    private Long userId;
    private String orderNo;
    private BigDecimal amount;
}

3. 创建`UserMapper`和`OrderMapper`接口，并继承`BaseMapper`接口：

public interface UserMapper extends BaseMapper<User> {
}

public interface OrderMapper extends BaseMapper<Order> {
}

4. 创建`UserAndOrderVO`类，用于封装查询结果：

@Data
public class UserAndOrderVO {
    private Long userId;
    private String userName;
    private Integer userAge;
    private String userEmail;
    private Long orderId;
    private String orderNo;
    private BigDecimal orderAmount;
}

5. 创建`UserAndOrderMapper`接口，用于实现连表分页查询：

public interface UserAndOrderMapper extends BaseMapper<UserAndOrderVO> {
    List<UserAndOrderVO> selectUserAndOrderPage(IPage<UserAndOrderVO> page, @Param("orderByField") String orderByField, @Param("userId") Long userId);
}

6. 在`UserAndOrderMapper.xml`中实现具体的查询逻辑：

<select id="selectUserAndOrderPage" resultMap="UserAndOrderVO">
    SELECT u.id AS userId, u.name AS userName, u.age AS userAge, u.email AS userEmail, o.id AS orderId, o.order_no AS orderNo, o.amount AS orderAmount
    FROM user u
    INNER JOIN `order` o ON u.id = o.user_id
    <if test="userId != null">
        WHERE u.id = #{userId}
    </if>
    ORDER BY ${orderByField} DESC
</select>

7. 在业务逻辑中调用`UserAndOrderMapper`接口中的方法，实现分页查询：

@Service
public class UserAndOrderService {
    @Autowired
    private UserAndOrderMapper userAndOrderMapper;

    public IPage<UserAndOrderVO> selectUserAndOrderPage(Page<UserAndOrderVO> page, String orderByField, Long userId) {
        return userAndOrderMapper.selectUserAndOrderPage(page, orderByField, userId);
    }
}

以上就是使用Mybatis-Plus实现连表分页查询的全部代码。