springboot+mybatis-plus实现两个数据库的两张表相连查询数据

可以使用Mybatis-Plus提供的多数据源配置，然后在查询时指定使用哪个数据源。以下是示例代码：

1. 配置多数据源

在application.yml文件中配置两个数据源：

spring:
  datasource:
    master:
      driver-class-name: com.mysql.jdbc.Driver
      url: jdbc:mysql://localhost:3306/db1?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=UTC
      username: root
      password: root
    slave:
      driver-class-name: com.mysql.jdbc.Driver
      url: jdbc:mysql://localhost:3306/db2?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=UTC
      username: root
      password: root

在配置类中添加多数据源配置：

@Configuration
@MapperScan(basePackages = "com.example.mapper")
public class DataSourceConfig {

    @Bean("master")
    @ConfigurationProperties("spring.datasource.master")
    public DataSource masterDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean("slave")
    @ConfigurationProperties("spring.datasource.slave")
    public DataSource slaveDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean
    public SqlSessionFactory sqlSessionFactory(@Qualifier("master") DataSource masterDataSource,
                                                 @Qualifier("slave") DataSource slaveDataSource) throws Exception {
        SqlSessionFactoryBean sessionFactory = new SqlSessionFactoryBean();
        sessionFactory.setDataSource(routingDataSource(masterDataSource, slaveDataSource));

        ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
        sessionFactory.setMapperLocations(resolver.getResources("classpath*:mapper/**/*.xml"));

        return sessionFactory.getObject();
    }

    @Bean
    public DataSource routingDataSource(@Qualifier("master") DataSource masterDataSource,
                                         @Qualifier("slave") DataSource slaveDataSource) {
        Map<Object, Object> dataSourceMap = new HashMap<>();
        dataSourceMap.put("master", masterDataSource);
        dataSourceMap.put("slave", slaveDataSource);

        RoutingDataSource routingDataSource = new RoutingDataSource();
        routingDataSource.setDefaultTargetDataSource(masterDataSource);
        routingDataSource.setTargetDataSources(dataSourceMap);

        return routingDataSource;
    }
}

2. 实现两个表的关联查询

在mapper中编写SQL语句，可以使用Mybatis-Plus提供的Wrapper类来实现多条件查询。

@Mapper
public interface UserMapper extends BaseMapper<User> {

    @Select("SELECT * FROM user u JOIN order o ON u.id = o.user_id WHERE u.id = #{userId}")
    @Results({
            @Result(column = "id", property = "id"),
            @Result(column = "username", property = "username"),
            @Result(column = "id", property = "orders", many = @Many(select = "com.example.mapper.OrderMapper.selectByUserId")),
    })
    User selectUserWithOrders(@Param("userId") Long userId);
}

@Mapper
public interface OrderMapper extends BaseMapper<Order> {

    @Select("SELECT * FROM order WHERE user_id = #{userId}")
    List<Order> selectByUserId(Long userId);
}

在service中调用mapper方法即可实现两个表的关联查询。

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    private UserMapper userMapper;

    @Override
    public User getUserWithOrders(Long userId) {
        return userMapper.selectUserWithOrders(userId);
    }
}