Mybatis-plus example code to implement multi-table query

There are usually several ways to implement multi-table query in MyBatis-Plus:

1. Use annotations for multi-table query

You can use it in the Mapper interface@Select Annotation is used to write SQL query statements to implement multi-table query. For example, if you want to query user information and corresponding area names based on user ID, you can write it like this:

@Mapper
@Repository
public interface UserMapper extends BaseMapper<User> {
    @Select("select user.* ,area.area_name from user,area " +
            "where user.area_id =  and  = #{id}")
    User getUserById(int id);
}

2. Use MyBatis-Plus Join Extension

MyBatis-Plus Join is an extension library that provides the ability to check multiple tables. You can use it by adding dependencies:

<dependency>
    <groupId></groupId>
    <artifactId>mybatis-plus-join</artifactId>
    <version>1.4.5</version>
</dependency>

Then, you can useJoinLambdaQueryWrapperTo construct the conditions for multi-table joint search. For example, query user information for each order:

import ;
import ;
import ;
public class OrderService {
    @Autowired
    private OrderMapper orderMapper;
    public List<UserOrderDTO> getOrderWithUser() {
        MPJLambdaWrapper<Order> wrapper = new MPJLambdaWrapper<>();
        ()        // Query all fields of the Order table               .select(User::getName)         // Query the Name field of the User table               .leftJoin(, User::getId, Order::getUserId);  // Use left join to connect        return (, wrapper);
    }
}

3. Use XML configuration files to perform multi-table query

Another approach is to define SQL statements for multi-table queries in the XML configuration file of Mapper. For example:

<mapper namespace="">
    <resultMap  type="">
        <result property="userName" column="name"/>
        <result property="userAge" column="age"/>
        <!-- Other field mappings -->
    </resultMap>
    <select  resultMap="orderMap">
        select o.order_id, o.user_id, o.goods_name, o.goods_price, , 
        from t_order as o left join t_user as u on o.user_id = 
    </select>
</mapper>

These methods provide flexible ways to implement multi-table query, and you can choose the appropriate implementation method according to the specific needs of the project.

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