mysql - sql嵌套顺序

Question

这是对这篇文章的澄清:

SQL select only rows with max value on a column

在接受的答案中，嵌套查询是用于最大计算的查询，外部查询连接到该查询。我尝试颠倒顺序，但遇到语法错误。

查询:

(SELECT id, MAX(rev) mrev
FROM docs 
GROUP BY id) b
join (select id, rev, content from docs) d
on b.id = d.id and d.rev = b.rev

我遇到的错误是这样的:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'b join (select id, rev, content from docs) d on b.id = d.id and d.rev = b.rev' at line 3

这里的顺序重要吗？

这是链接: http://sqlfiddle.com/#!9/a6c585/64570

Answer 1

您可以像这样编写该查询。

SELECT d.*
FROM
(
  SELECT id, MAX(rev) AS maxrev
  FROM docs 
  GROUP BY id
) b
JOIN docs AS d
ON (b.id = d.id AND d.rev = b.maxrev)

注意它如何从子查询中选择最大转速。而子查询只是简单地连接到表。

另一种写法:

select d.*
from docs d
join (
  select id, max(rev) maxrev
  from docs
  group by id
) b
on b.id = d.id and b.maxrev = d.rev

或者如果你敢于使用 EXISTS :

SELECT *
FROM docs AS d
WHERE EXISTS (
  SELECT 1
  FROM docs AS b
  WHERE b.id = d.id 
  GROUP BY b.id
  HAVING d.rev = MAX(b.rev)
);