SQL 練習題

[Yen Lai]

SQL 練習題

tags: Database SQL

👇 Database 👇

Tables
Authors	id	name
Books	id	title	author_id	publisher_id
Publishers	id	name
Readers	id	name	department	gender
Readerships	id	reader_id	book_id

題目

1. 總共有幾本書在資料庫中？

select count(*)
from books;

--answer: 100

2. 總共有幾位作者在資料庫中？

select count(*)
from authors;

--answer: 59

3. 總共有幾家出版社在資料庫中？

select count(*)
from publishers;

--answer: 33

4. 一位作者的書只算一次，總共有幾本不同作者的書？

select count(distinct author_id)
from books;

--answer: 59

5. 平均每位讀者借過幾次書（包含重覆的書）？

select count(*)/count(distinct reader_id)
from readerships;

--answer: 50

==6. 平均每位讀者借過幾本不重覆的書？==

select count(distinct book_id, reader_id)/count(distinct reader_id)
from readerships;

--answer: 39.14

7. 借閱次數最多的讀者（包含重覆借同一本書），總共借了幾次書？

select r.name, count(rs.book_id)
from readerships as rs
join readers as r
    on r.id = rs.reader_id
group by r.name
order by count(rs.id) desc limit 1;

--answer: 66

8. 借閱記錄最多的讀者的名字是（包含重覆借同一本書）？

select r.name, count(rs.book_id)
from readerships as rs
join readers as r
    on r.id = rs.reader_id
group by r.name
order by count(rs.id) desc limit 1;

--answer: Cathey Gardella

9. 借過最多本不同書的讀者，總共借過幾本不同的書？

select r.name, count(distinct rs.book_id)
from readerships as rs
join readers as r
    on r.id = rs.reader_id
group by r.name
order by count(rs.book_id) desc limit 1;

--answer: 47

10. 借過最多本不同書的讀者，名字是？

select r.name, count(distinct rs.book_id) as cnt
from readers as r
join readerships as rs
    on r.id = rs.reader_id
group by r.name
order by cnt desc limit 1;

--answer: Tashia Piccard

11. 所有女性讀者借書的總次數是？

select count(rs.id)
from readerships as rs
join readers as r
    on r.id = rs.reader_id
where r.gender = 'F';

--answer: 2570

12. 已知共有50位男性讀者，所有男性讀者借書的平均次數是？

select count(rs.id)/50
from readerships as rs
join readers as r
    on r.id = rs.reader_id
where r.gender = 'M';

--answer: 48.6000

==13. 哪一本書最熱門？（被最多不同讀者借過）==

注意 group by 要以 books.id 來分組

select b.title, count(distinct rs.reader_id) as cnt
from readerships as rs
join books as b
    on b.id = rs.book_id
group by b.id
order by cnt desc limit 1;

--answer: Harry Potter and the Half-blood Prince

==14. 哪一本書最花時間消化？（每位讀者平均借閱次數最多）==

注意 group by 要以 books.id 來分組

select b.title, count(rs.id)/count(distinct rs.reader_id) as cnt
from readerships as rs
join books as b
    on rs.book_id = b.id
group by b.id
order by cnt desc limit 1;

15. 哪一位讀者閱讀速度最快（平均每一本書的借閱次數最少）？

select r.name, count(rs.id)/count(distinct rs.book_id) as cnt
from readerships as rs
join readers as r
    on r.id = rs.reader_id
group by r.name
order by cnt asc limit 1;

--answer: Meryl Nguyen

16. 哪一家出版社出最多書？

select p.name, count(b.publisher_id) as cnt
from books as b
join publishers as p
    on p.id = b.publisher_id
group by p.id
order by cnt desc limit 1;

--answer: Bloomsbury Pub PLC

==17. 哪一家出版社旗下的書，被最多（不重覆）讀者借過？==

以同時具有兩個表格外鍵的「中間表格」，當做第一個出現的表格，後面會較好寫指令。

select p.name, count(distinct rs.reader_id) as cnt
from readerships as rs
join books as b
    on b.id = rs.book_id
join publishers as p
    on p.id = b.publisher_id
group by p.id
order by cnt desc limit 1;

--answer: Bloomsbury Pub PLC

18. 有些書是匯編而成的，所以沒有列作者。這樣子的書共有幾本？

select count(*)
from books
where author_id is null;

--answer: 4

19. 最多產（寫過最多本書）的作家是誰？

select a.name, count(b.id)
from books as b
join authors as a
    on a.id = b.author_id
group by a.name
order by count(b.id) desc limit 1;

--answer: Rowling, J. K.

20. 有幾本書書名中含有 time 這個字？（不區分大小寫）

select count(*)
from books
where title like '%time%';

-- answer: 4