mybatis实现跨库多表查询

需求：查询用户名具体负责的项目的名称
已有数据库表如下所示，二者通过项目ID关联

test数据库的user表
test1数据库的project表

需要实现的SQL查询

select user.id,user.name,projectName from test.user left join test1.project on user.`projectId`=project.id;

查询结果 查询结果

实现：核心思想，手动新建Bean与Mapper。

1.自动生成代码model与mapper，包括test.user test1.project
2.新建一个bean命名为UserPro，其属性为上述所需的3个字段

public class UserPro {
    private Integer id;
    private String name;
    private String projectname;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getProjectname() {
        return projectname;
    }

    public void setProjectname(String projectname) {
        this.projectname = projectname;
    }

}

3.新建一个mapper命名为UserProMapper,返回新建的bean: UserPro

@Repository
public interface UserProMapper {

    @Select("select user.id,user.name,projectName from test.user left join test1.project on user.`projectId`=project.id limit 1")
    UserPro findProName();
}

4.基本已经实现核心的内容了，接下来就是用controller实现调用关系

@Controller
public class testController {

    @Autowired
    private StuClassMapper stuClassMapper;
    @Autowired
    private UserMapper userMapper;

    @Autowired
    private UserProMapper userProMapper;

    @RequestMapping("/test")
    @ResponseBody
    public String  getStuClass(){
        User user=userMapper.findByUserId();
        System.out.println(user.getName());
        return user.getName();
    }

    @RequestMapping("/test2")
    @ResponseBody
    public String  getStuClassByJoin(){

        System.out.println(userMapper.findByUserIdJoin());
        return "userMapper.findByUserIdJoin()";
    }

    @RequestMapping("/test3")
    @ResponseBody
    public UserPro getProName(){

        UserPro userPro=userProMapper.findProName();
        return userPro;
    }

    @RequestMapping("/test4")
    @ResponseBody
    public String  findUserNameByUserIdJoin(){
        return userMapper.findUserNameByUserIdJoin();
    }

}

5.启动项目，在浏览器输入：http://localhost:8091/test3

执行结果
大家也发现了，controller中有很多的路由匹配，这是模拟了其他若干场景。另外为了方便，Mapper返回的查询结果条数限制为1：

# test
select * from test.user where id in(select userId from test1.stuClass) limit 1;
# test2
select count(*) from test.user left join test1.stuClass on user.id=stuClass.userId
# test3 
select user.id,user.name,projectName from test.user left join test1.project on user.`projectId`=project.id;
# test4
select stuClass.userId from test.user left join test1.stuClass on user.id=stuClass.userId limit 1
# ？
select * from test.user left join test1.stuClass on user.id=stuClass.userId;

源码链接：https://gitee.com/neimenggudaxue/MultityTablesCurd.git