Question

我需要返回最常购买的书的书名。如果购买了多本相同金额的书，则会收到错误消息：

SELECT title
FROM books JOIN orderitems
USING (isbn) WHERE isbn=
    (SELECT isbn
    FROM orderitems
    HAVING SUM (quantity) =
        (SELECT MAX(SUM(quantity))
        FROM orderitems
        GROUP BY isbn)
    GROUP BY isbn)
GROUP BY title;

Answer 1

您的子查询返回不支持=运算符的多行，因此我使用了in

SELECT title
FROM books JOIN orderitems
USING (isbn) WHERE isbn in
    (SELECT isbn
    FROM orderitems
    group by isbn
    HAVING SUM (quantity) =
        (SELECT MAX(SUM(quantity))
        FROM orderitems
        GROUP BY isbn
        )
    )

Answer 2

使用窗口功能！

SELECT title
FROM (SELECT isbn, title, SUM(quantity) as quantity,
             RANK() OVER (ORDER BY SUM(quantity) DESC) as seqnum
      FROM books b JOIN
           orderitems oi
           USING (isbn)
      GROUP BY isbn, title
     ) t
WHERE seqnum = 1;

Answer 3

您要按订单数量对书籍进行排名，并且仅显示排名最高的书籍（数量最大的书籍排在第一位）。有窗口功能RANK用于对行进行排名。

您只想选择书名，因此从书中选择书名，并在WHERE子句中应用您的条件。

select title
from books
where (isbn, 1) in -- ISBN ranked #1
(
  select isbn, rank() over (order by sum(quantity) desc)
  from orderitems
  group by isbn
);

如何使我的查询避免“单行子查询返回多个行”错误

3 个答案: