mysql无法调用动态表名的问题

这周写动态调用的时候遇到了一个问题就是我可以将动态值赋值给字段，却无法复制给表名

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.itheima.core.dao.UserDao" >
    <!-- 查询用户 -->
    <select id="findUser" parameterType="String" resultType="user">
	    select *
	    from  #{usercode}
	    where user_code = #{usercode}
	    and user_password =#{password}

    </select>
</mapper>

表名和usercode都是bus

但是这样却无法访问但将from后面的变量变成bus却又可以使用，我就很奇怪
最后发现是${}和#{}的问题，修改成这样就好了

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.itheima.core.dao.UserDao" >
    <!-- 查询用户 -->
    <select id="findUser" parameterType="String" resultType="user">
	    select *
	    from  ${usercode}
	    where user_code = #{usercode}
	    and user_password =#{password}

    </select>
</mapper>

具体区别可以去看这个网址：http://www.cnblogs.com/heyonggang/p/5953070.html