mybatis多表关联

1.比如我有两个一个是菜单表t_menu,一个是权限表t_jurisdiction.表结构如下：

 

2.我想要将这两个表关联，查询特定的role_id下的菜单情况，这也是我们经常用在权限管理系统中的做法，将角色与菜单关联起来。大概的sql语句如下

select t1.menu_id as menuId,t1.p_menu_id as pMenuId, t1.menu_name as menuName,ifnull(t2.jurisdiction_id,0) as jurisdictionId from t_menu t1 left join (select jurisdiction_id，menu_id from t_jurisdiction where role_id=1)t2 on t1.menu_id=t2.menu_Id

该查询会出现类似如下的结果。

 其中jurisdictionId为0表示该用户还没选中这个菜单作为它的权限。

3.在mybatis中实现

首先，我们定义一个包含了上述字段的resultMap，将它的类型设为java.util.Map，如下所示：

<!-- 包含了菜单信息的权限列表 -->
    <resultMap id="ResultMapIncludeJurisdiction" type="java.util.Map">
        <id column="menu_id" property="menuId" jdbcType="INTEGER" />
        <result column="p_menu_id" property="pMenuId" jdbcType="INTEGER" />
        <result column="menu_name" property="menuName" jdbcType="VARCHAR" />
        <result column="jurisdiction_id" property="jurisdictionId" jdbcType="INTEGER" />
    </resultMap>

然后，定义一个sql语句变量，如下

<sql id="Column_List_include_jurisdiction">
        t1.menu_id, t1.p_menu_id ,t1.menu_name,ifnull(t2.jurisdiction_id,0) as jurisdiction_id
    </sql>

接下来就是查询语句

<!-- 通过 roleId获取其菜单列表，并标明是否有选中某项菜单-->
    <select id="selectJurisdictionByRoleId" parameterType="java.lang.Integer" resultMap="ResultMapIncludeJurisdiction">
        select
        <include refid="Column_List_include_jurisdiction" />
        from t_menu t1 left join (select jurisdiction_id,menu_id from t_jurisdiction where role_id=#{roleId,jdbcType=INTEGER}) t2 on t1.menu_id=t2.menu_id
    </select>

然后在Service层调用这个查询语句就可以了

@Service
public class JurisdictionService {
    @Resource(name="baseDao")
    private BaseDao<TJurisdiction,Integer>baseDao;
    public List<Map<String,Object>>getJurisdictionByRoleId(int roleId){
        return baseDao.findMapListByPm("TJurisdictionMapper.selectJurisdictionByRoleId", roleId);
    }
    public static void main(String args[]){
          ApplicationContext ctx = new ClassPathXmlApplicationContext(
                    "applicationContext.xml");
        JurisdictionService jurisdictionService=(JurisdictionService) ctx.getBean("jurisdictionService");
        System.out.println(jurisdictionService.getJurisdictionByRoleId(1));
    }
}

上述的main方法是用于测试的，测试结果符合我们的需要，关于baseDao的编写，可参考博客其他文章。

<sql id="Column_List_include_jurisdiction">
        t1.menu_id, t1.p_menu_id,t1.menu_name,ifnull(t2.jurisdiction_id,0)
    </sql>