僅在哪裏查詢？

Question

在sqlite中是否只有'only'這樣的關鍵字？我必須寫一個查詢來顯示只保留紅船的水手名稱。所以我當前的查詢是：

select s.sname 
from sailor s, boat b, reservation r 
where s.sname = r.sname 
and b.bname = r.bname 
and b.color = 'red'; 

與上面的查詢，還顯示誰有權紅+其他綵船水手名稱的問題。我的查詢結果：

a reserve red boat 
a reserve green boat 
b reserve red boat 

但它應該只顯示b，因爲他只保留紅船。

Answer 1

你可以使用NOT EXISTS從句來過濾水手只有red船。

select r.sname 
from reservation r 
join boat b 
on b.bname = r.bname 
and b.color = 'red' 
and not exists (select 1 from reservation r2 
       join boat b2 
       on r2.bname = b2.bname 
       and r2.sname = r.sname 
       and b2.color <> 'red') 

Answer 2

存在多個選項。您可以使用一個NOT IN運營商像

select s.sname 
from sailor s, boat b, reservation r 
where s.sname = r.sname 
and b.bname = r.bname 
and b.color NOT IN (select distinct color from boat where color <> 'red'); 

而且，不是每一個水手將預留一條船。所以在這種情況下，你最好用LEFT JOIN而不是INNER JOIN。此外，我認爲你的意思是做一個group by水手名稱像下面這樣

select s.sname 
from sailor s, 
left join reservation r on s.sname = r.sname 
left join boat b on b.bname = r.bname 
group by s.sname 
having count(*) <= 1 and b.color = 'red'