text: 1. 统计每位作家出版图书的平均价格
  select author,avg(price) from book group by author;
  2. 统计每个出版社出版图书数量
  select publishing_company,count(title) from book group by publishing_company;
  3. 查看总共有多少个出版社
  select count(distinct publishing_company) from book;
  4. 筛选出那些出版过超过50元图书的出版社，并按照其出版图书的平均价格降序排序
  select publishing_company,avg(price) from book
  group by publishing_company
  having max(price)>50
  order by avg(price) desc;
  5. 统计同一时间出版图书的最高价格和最低价格
  select publish_time,max(price),min(price) from book group by publish_time;