仅在哪里查询？

Question

Is there keyword like 'only' in sqlite? I have to write a query to display the sailor name who have reserved only red boat. So my current query is:

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

The problem with the query above is it also display the sailor name who reserve red + other color boat. My query result:

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

but it should display only b since he only reserve red boat.

Answer 1

you can use NOT EXISTS clause to filter the sailor with only red boat.

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

There are multiple option present. You can use a NOT IN operator like

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'); 

Also, not every sailor will reserve a boat. So in that case, you better off for a LEFT JOIN instead of INNER JOIN . Also, I think you meant to do a group by sailor name something like below

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'