[英]SQL: combine update with group-by?
我有两张桌子:
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
现在我想用enrichments.na
的值更新events.na
列,其中mac
匹配, timestamp
是该特定mac
的最大timestamp
。
对于此示例,结果将是:
abc | na3
abc | na3
def | na4
最有效的方法是什么?
谢谢!
这应该使用公用表表达式与SQL Server一起使用
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
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
)
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