SQL：將更新與組合更新？

Question

我有兩個表：

events：

mac | na 
---------- 
abc | (null) 
abc | (null) 
def | (null) 

enrichments：

mac | na | timestamp 
--------------------- 
abc | na1 | 1 
abc | na2 | 2 
abc | na3 | 3 
def | na4 | 1 

現在我想在enrichments.na更新events.na列與值，其中mac比賽，並timestamp是該特定mac的最大timestamp。

在這個例子中，結果將是：

abc | na3 
abc | na3 
def | na4 

什麼是做到這一點的最有效方法是什麼？

謝謝！

Answer 1

UPDATE events AS e 
SET na = 
    (SELECT n.na 
     FROM enrichments AS n 
     WHERE n.mac = e.mac 
     ORDER BY n.timestamp DESC 
     FETCH FIRST 1 ROWS ONLY 
    ) 

Answer 2

這應該與SQL Server使用公共表表達式

create table events (mac nvarchar(3), na nvarchar(3)) 
insert into events values ('abc',null); 
insert into events values ('abc',null); 
insert into events values ('def',null); 

create table enrichments (mac nvarchar(3), na nvarchar(3), timestamp int) 
insert into enrichments values ('abc','na1',1) 
insert into enrichments values ('abc','na2',2) 
insert into enrichments values ('abc','na3',3) 
insert into enrichments values ('def','na4',1) 

with latestEnrichment (mac, latestTimestamp) as 
    (select mac, MAX(timestamp) 
    from enrichments 
    group by mac) 
update events 
set  na = en.na 
from events e join enrichments en on e.mac = en.mac 
     join latestEnrichment le on en.timestamp = le.latesttimestamp and 
     en.mac = le.mac