MyBatis-Plus-Join联表查询

MyBatis-Plus-Join官网

1：添加依赖

<dependency><groupId>com.github.yulichang</groupId><artifactId>mybatis-plus-join-boot-starter</artifactId><version>1.5.3</version>
</dependency>

2：原来写法的Mapper类，继承BaseMapper（其他全部不用修改）

@Mapper
public interface UserMapper extends BaseMapper<User> {
}

2-1：修改为的继承MPJBaseMapper

@Mapper
public interface UserMapper extends MPJBaseMapper<User> {
}

3：例子

MPJLambdaWrapper<DoorControllerSettingsDoorInfoDO> doorControllerSettingsDoorInfoDOMPJLambdaWrapper = new MPJLambdaWrapper<DoorControllerSettingsDoorInfoDO>().selectAll().leftJoin(DoorControllerSettingsDO.class, DoorControllerSettingsDO::getControllerNumber, DoorControllerSettingsDO::getControllerNumber);
doorControllerSettingsMapper.selectJoinList(DoorControllerSettingsDO2.class, doorControllerSettingsDO2MPJLambdaWrapper);

4：对于selectCollection和selectAssociation的使用

4-1：User 实体类

@Data
@TableName("user")
public class User {@TableIdprivate Long id;private String username;private String password;private Integer age;private String email;
}

4-2：Address 实体类

@Data
@TableName("address")
public class Address {@TableIdprivate Long id;private Long userId;  // 外键，关联user.idprivate String province;private String city;private String district;private String detail;
}

4-3：UserDTO 数据传输对象

@Data
public class UserDTO {private Long id;private String username;private String password;private Integer age;private String email;// 使用List接收一对多关系private List<Address> addressList;
}

4-4：userMapper代码查询

MPJLambdaWrapper<User> wrapper = new MPJLambdaWrapper<User>().selectAll(User.class)//全部映射 不用考虑字段名重复问题(比如 id), 会对重复列自动添加别名.selectCollection(Address.class, UserDTO::getAddressList).leftJoin(Address.class, Address::getUserId, User::getId);
List<UserDTO> dtoList= userMapper.selectJoinList(UserDTO.class, wrapper);

4-5：生成的SQL语句，基于MPJLambdaWrapper代码，MyBatis-Plus-Join会生成如下SQL：

SELECT user.id AS user_id,user.username AS user_username,user.password AS user_password,user.age AS user_age,user.email AS user_email,address.id AS address_id,address.user_id AS address_user_id,address.province AS address_province,address.city AS address_city,address.district AS address_district,address.detail AS address_detail
FROM user
LEFT JOIN address ON address.user_id = user.id

4-6：执行结果生成sql处理，MyBatis-Plus-Join会自动处理结果集，将数据映射为：

	List<UserDTO> dtoList = [{"id": 1,"username": "张三","password": "123456","age": 25,"email": "[e-mail]","addressList": [{"id": 1,"userId": 1,"province": "北京","city": "北京市","district": "海淀区","detail": "中关村大街1号"},{"id": 2,"userId": 1,"province": "上海","city": "上海市","district": "浦东新区","detail": "张江高科技园区"}]},
]

注意

//使用如下方法
JoinWrappers.lambda(DoorControllerSettingsDoorInfoDO.class)//替换
new MPJLambdaWrapper<DoorControllerSettingsDoorInfoDO>()//使用如下方法
Wrappers.lambda(DoorControllerSettingsDoorInfoDO.class)//替换
new LambdaWrapper<DoorControllerSettingsDoorInfoDO>()