mybatic 结果为null，返回0

返回空容易产生异常，当值为null时，返回为0的写法

<select id="getBookWordCountBookPssAndChapterLockOrPass" resultType="LONG" parameterType="STRING">
        select  COALESCE(sum(word_count),0) as word_count
        from t_news_count
        WHERE
        chapter_id  in
        ( SELECT chapter_id
          from t_content_chapter
          WHERE
          book_id = #{book_id,jdbcType=VARCHAR}
          AND
          ( status = 3 OR status = 1) )
        AND
        ( 3 = (SELECT current_state
               FROM t_content_metadata
               WHERE  book_id = #{book_id,jdbcType=VARCHAR} )
        )
    </select>