mybatis 用cast when 批量更新

xml中书写的方式

<update id="batchUpdate" parameterType="list">
         update songs
            <trim prefix="set" suffixOverrides=",">
                 <trim prefix="path =case" suffix="end,">
                     <foreach collection="list" item="i" index="index">
                             <if test="i.songid!=null">
                                  when songid=#{i.songid} then #{i.path}
                             </if>
                     </foreach>
                  </trim>
             </trim>
            where songid in 
            <foreach collection="list"  item="i" index="index" open="(" separator="," close=")">
              #{i.songid}
          </foreach>
    </update>

Mapper中方法

void batchUpdate(List<Songs> recordList);

打印出来的sql格式

UPDATE songs 
SET path =
CASE  SONGID 
      WHEN '10800537' THEN    'C:/songs/10800537.mpg'
      WHEN '10800118' THEN    'C:/songs/10800118.mpg'
END    
WHERE   SONGID IN ('10800537','10800118')