SQL最大条件加入

Question

因此，我们有3个相关表格：

• book与属性title, numberofpages
• person与last name和id
• writes与author（= person.id）和book（= book.title）

所以我们想找出每个作者的姓氏和他的书页数最高的页码。

我们尝试了这一点：

SELECT lastname, numberofpages 
FROM book, person, writes 
WHERE person.id=writes.author AND book.title=writes.book 

这将返回：

author A number of pages of his book 1 
author A number of pages of his book 2 
author B number of pages of his book 1 

等等...

我们怎样才能使它每次只包含笔者给出一个排只有最高数量的书的页数？我们用max(numberofpages)尝试了各种子查询，但无法使其正常工作。

Answer 1

您必须使用group by条款是这样的：

select p.lastname, max(w.numberofpages) 
from writes w 
inner join person p on p.id = w.author 
inner join books b on b.title = w.book 
group by p.lastname, p.id 

Answer 2

您需要组由作者，并挑选max：

SELECT person.lastname, max(book.numberofpages) 
FROM book, person, writes 
WHERE person.id=writes.author AND book.title=writes.book 
GROUP BY person.lastname, person.id 

你也应该考虑使用现代化的连接语法，并保存有关使用不同的ID，但相同的姓氏作者信息：

SELECT person.lastname, max(book.numberofpages) 
FROM book 
JOIN writes ON book.title=writes.book 
JOIN person ON person.id=writes.author 
GROUP BY person.id, person.lastname 

Answer 3

您必须使用GROUP BY：

SELECT person.lastname, max(writes.numberofpages) 
FROM book, person, writes 
WHERE person.id=writes.author AND book.title=writes.book 
GROUP BY person.lastname, person.id