如何使用2個表格獲得某個類別中最昂貴的商品？

Question

我試圖去顯示只有最昂貴的項目類別。

我試過了，並且我已經查詢了所有類別和每個類別中最昂貴物品的價格，但是我不知道如何才能得到最昂貴的一個類別。

select categories.category ,max(purchase_price) as dyrast_bok 
from categories 
inner join books on categories.category_id = books.category_id 
group by categories.category; 

的表格：

CATEGORIES (category_id (PK), category) 

BOOKS (book_id (PK), title, publisher_id (FK), published_year, 
purchase_price, category_id (FK),purchase_date, pages, 
book_type_id (FK)) 

Answer 1

您可以嘗試max(purchase_price) desc訂購併佔據第一位：

select * 
from (your query with order by max(purchase_price) desc) 
limit 1 

Answer 2

你只需要使用別名，因此您可以在重新使用它們GROUP BY部分，並使用top 1只有在您訂購max(b.purchase_price)時才能獲得最高結果。

這是應該的查詢：

select top 1 c.category , max(b.purchase_price) as dyrast_bok 
from categories c 
inner join books b on categories.category_id = books.category_id 
order by dyrast_bok 
group by c.category; 

Answer 3

select categories.category ,purchase_price as dyrast_bok 
from categories 
inner join books on categories.category_id = books.category_id 
where purchase_price in (select max(purchase_price) from books) 

Answer 4

簡單的方法：有一個子查詢，返回最大purchase_price。

select c.category, b.purchase_price as dyrast_bok 
from categories c 
inner join books b on c.category_id = b.category_id 
where b.purchase_price = (select max(purchase_price) from books) 

如果系領帶，會退還兩本書！

替代解決方案：

select c.category, b.purchase_price as dyrast_bok 
from categories c 
inner join books b on c.category_id = b.category_id 
order by b.purchase_price desc 
fetch first 1 row 

將不包括領帶，只有一行將被退回。 （AFAIK Postgresql不支持FETCH FIRST WITH TIES。）