1.比如我有兩個一個是菜單表t_menu,一個是權限表t_jurisdiction.表結構如下：

2.我想要將這兩個表關聯，查詢特定的role_id下的菜單情況，這也是我們經常用在權限管理系統中的做法，將角色與菜單關聯起來。大概的sql語句如下

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

該查詢會出現類似如下的結果。

其中jurisdictionId為0表示該用戶還沒選中這個菜單作為它的權限。

3.在mybatis中實現

首先，我們定義一個包含了上述字段的resultMap，將它的類型設為java.util.Map，如下所示：

<!-- 包含了菜單信息的權限列表 -->
    <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>

然後，定義一個sql語句變量，如下

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

接下來就是查詢語句

<!-- 通過 roleId獲取其菜單列表，並標明是否有選中某項菜單-->
    <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>

然後在Service層調用這個查詢語句就可以了

@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方法是用於測試的，測試結果符合我們的需要，關於baseDao的編寫，可參考博客其他文章。

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

mybatis多表關聯