sql - 如何将表中的值更新为其他行的 MAX？

Question

考虑这张表：

Declare @Content table (id int, ParentId int, CreatedOn DateTime, LastCommentOn DateTime)

insert into @Content values 
(1, null, GETDATE() - 10, '2001-12-01'),
(2, 1, GETDATE() - 9, GETDATE() - 8),
(3, 1, GETDATE() - 8, GETDATE() - 7),
(4, 1, GETDATE() - 7, GETDATE() - 6),
(5, null, GETDATE() - 6, '2001-12-01'),
(6, 5, GETDATE() - 5, GETDATE() - 4),
(7, 5, GETDATE() - 4, GETDATE() - 3),
(8, null, GETDATE() - 3, '2001-12-01'),
(9, 8, GETDATE() - 2, GETDATE() - 1)

我想将所有主要内容（由 ParentId 标识为 null）更新为对该内容的最后评论的 CreatedOn 日期。

我试过了

update @Content m 
set LastCommentOn = MAX(select CreatedOn from @Content c where c.ParentId = m.Id) 
where ParentId is null and LastCommentOn = '2001-12-01'

和

update @Content 
set LastCommentOn = MAX(select CreatedOn from @Content c where c.ParentId = m.Id) 
from @Content m
where ParentId is null and LastCommentOn = '2001-12-01'

但我不能让它做我想做的事..

请问如何在 MSSQL 中执行此操作？

（另外，在 mysql 上的查询是否相同？）

---

接受的答案在 MS SQL 上效果很好，但是在 MySql 上，我找不到在一个语句中执行此操作的方法，我不得不将查询分成两部分并更新..所以这对我在 mysql 上有用

SET SQL_SAFE_UPDATES=0;

Create temporary table tmpContentDates 

select Max(ParentId) as pid, Max(CreatedOn) as pd
From Content
where ParentId is not null
Group By ParentId; 

update Content as c
    inner join tmpContentDates d on c.Id = d.pid
set c.LastCommentedOn = d.pd
where ParentId is null
    and LastCommentedOn = '2001-12-01';

drop table tmpContentDates;

Answer 1

你有没有尝试过？

update @Content 
set LastCommentOn =  
(select MAX(CreatedOn) from @Content c where c.ParentId = m.Id) 
from @Content m
where ParentId is null and LastCommentOn = '2001-12-01'

Answer 2

你的 SQL 应该是

update @Content m 
set LastCommentOn = (select MAX(CreatedOn) from @Content c where c.ParentId = m.Id)
where ParentId is null and LastCommentOn = '2001-12-01'

假设 max 适用于 DateTime

Answer 3

update @Content 
set LastCommentOn = MAX(c.CreatedOn) 
from @Content m
inner join @Content c on c.ParentId = m.Id
where ParentId is null and LastCommentOn = '2001-12-01'