通过连接表最大值仅选择行

Question

我想只为u.classno,u.userno选择具有max b.enddate的行.但它不起作用.

select u.classno, u.userno, b.enddate
from libUser u
join book b on b.id = u.bookid
group by u.classno, u.userno
having b.enddate=max(b.enddate) //doesn't works

Answer 1

这是官方MySQL文档中的一篇优秀文章,但在那里只使用标准SQL,因此它可以应用于您正在使用的任何RDBMS.

行保持某一列的组最大值

任务:对于每篇文章,找到价格最贵的经销商或经销商.

这个问题可以通过像这样的子查询来解决:

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article);

前面的示例使用相关子查询,这可能是低效的(请参见第13.2.10.7节"相关子查询").解决问题的其他可能性是在FROM子句或LEFT JOIN中使用不相关的子查询.

不相关的子查询:

SELECT s1.article, dealer, s1.price
FROM shop s1
JOIN (
  SELECT article, MAX(price) AS price
  FROM shop
  GROUP BY article) AS s2
  ON s1.article = s2.article AND s1.price = s2.price;

LEFT JOIN:

SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price
WHERE s2.article IS NULL;

LEFT JOIN的工作原理是,当s1.price处于其最大值时,没有s2.price具有更大的值,并且s2行值将为NULL.

Answer 2

什么错了:

select u.classno, u.userno, MAX(b.enddate)
from libUser u
join book b on b.id = u.bookid
group by u.classno, u.userno