Mybatis多表查询的问题

<?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.group2.mapper.CommentMapper">
<resultMap id="commentmap" type="com.group2.vo.CommentVo">
	<association property="goods" column="goodsid" javaType="com.group2.po.Goods" select="selectCommentByGoodsId">
	</association>
</resultMap>	
	<select id="selectByUserid" resultMap="commentmap">
		SELECT userid,commentdetails,commentdate,commentlevel,goodsid,commentid
		FROM comment WHERE userid=#{userid}
	</select>
	<select id="selectCommentByGoodsId" parameterType="int" resultType="com.group2.po.Goods">
		SELECT goodsname,goodsphoto 
		FROM goods WHERE goodsid=#{goodsid}
	</select>
</mapper>

被用作查询的字段goodsid值为空了，查询结果如下。

CommentVo [commentid=6, goodsid=null, userid=2, commentdetails=大大, commentdate=2017-08-11, commentlevel=1, goods=Goods [goodsid=null, goodsname=老韩家凤梨酥, goodsdescription=null, goodsphoto=b532afbd-2166-4868-a5b6-1ac6b8038ec61.jpg,46803503-3b1a-42b1-a31a-3460a001fb972.jpg,d3c9bbbf-5788-4f31-975f-51ba7d0b8f263.jpg,ddf9603d-59b1-41dd-a8ae-1378d2cd01aa6.jpg,, goodsprice=0.0, goodscount=null, typeid=null, shopid=null, goodsstatus=null, goodsdate=null]]

可以看见第一个字段goodsid的值为空，第二个goodsid我没查所以为空这个我知道就不用说了。

问题就是怎么让第一个goodsid有值呢。