mybatis中表的关联

一、数据库中表的关联有三种形式

　　a)一对一

　　b)一对多(多对一)

　　c)多对多

二、mybatis实现多对一(两种方式)、这里我们用用户和部门进行举例实现(多个用户(员工)同属于一个部门、一个部门有多个用户(员工))

　　a)结果嵌套查询

　　dao接口:　　

List<User> selectAllUsers();

　　dao实现类:

@Override
    public List<User> selectAllUsers() {
        SqlSession sqlSession=null;
        List<User> userList = null;
        try {
            userList = MyBatisUtils.getSqlSesion("mybatis/mybatis.xml").selectList("edu.aeon.mybatis.entity.UserMapper.selectAllUsers");
        } catch (IOException e) {
            e.printStackTrace();
        }
        return userList;
    }

　　实体类和数据库的sql映射文件：(User.Mapper.xml)

<!-- ①多对一 :按结果嵌套-->
 <select id="selectAllUsers" resultMap="UserDepartment">
     select u.*,d.* from user u,department d where u.did = d.did
 </select>
 <resultMap type="User" id="UserDepartment">
     <id property="uid" column="uid"/>
     <result property="uname" column="uname"/>
     <result property="upw" column="upw"/>
     <association property="department" javaType="Department">
         <id property="did" column="did"/>
         <result property="dname" column="dname"/>
     </association>
 </resultMap>

　　测试类:　　

public static void testSelectAllUsers(){
        UserDao userDao =new UserDaoImpl();
        List<User> userList = userDao.selectAllUsers();
        for(User user:userList){
            System.out.println(user);
        }
    }

　　b)按查询嵌套

　　　　dao及实现同上

　　　　mybatis核心配置文件：

　　　　

<mappers>
 <!-- 引入实体类到数据库的sql映射文件 -->
 <mapper resource="edu/aeon/mybatis/entity/User.Mapper.xml"/>
 <mapper resource="edu/aeon/mybatis/entity/Deparment.Mapper.xml"/>
 </mappers>

　　　　实体类和数据库的sql映射文件：(User.Mapper.xml)

 <!-- ②多对一 :按查询嵌套-->
 <select id="selectAllUsers" resultMap="UserDepartment">
     select * from user
 </select>
 <resultMap type="User" id="UserDepartment">
    <association property="department" column="did" javaType="Department" select="edu.aeon.mybatis.entity.DepartmentMapper.selectDepartmentById"></association>
 </resultMap>

　　　　实体类和数据库的sql映射文件：(Deparment.Mapper.xml)

<mapper namespace="edu.aeon.mybatis.entity.DepartmentMapper">
 <select id="selectDepartmentById" parameterType="int" resultType="Department">
     select * from department where did=#{did}
 </select>
</mapper>

　　测试同上！