1. 统计读者总人数。

select count(distinct reader_number)

from reader;

2. 统计计算机类图书的总藏书量。

select sum(book_total)

from book

where category = '计算机';

3. 查询机械工业出版社图书的最高价及最低价。

select max(price),min(price)

from book

where publisher = '机械工业出版社';

4. 统计每位读者的借书数量，列出读者号及借书数量。

select reader_number,count(book_number)

from borrow_book

group by reader_number;

5. 求机械工业出版社出版的各类图书的平均定价，列出图书类别及平均定价。

select category ,avg(price)

from book

where publisher = '机械工业出版社'

group by category;

6. 查询图书平均价格在20-30的出版社名称及平均定价。

select publisher,avg(price) as avg_price

from book

group by publisher

having avg_price between 20 and 30;

7. 查找这样的图书类别：要求类别中最高的图书定价不低于全部按类别分组的图书的平均定价的2倍，列出图书类别，最高定价及平均定价的2倍。

select category,MAX(price),AVG(price)*2

from book

group by category

having MAX(price)>= all(

       select AVG(price)*2

       from book

       group by category

);

8. 统计各类图书的总藏书量，列出图书类图及总藏书量。

select category,SUM(book_total)

from book

group by category;