SQL Server：通过匹配不同列中的值，用同一列中的值替换Null值

Question

I have a table in SQL Server with some null values in column "date":

platform   date         id
---------------------------
web        2018-10-10   1
mob                     1
mob                     1
web        2018-10-15   2
mob                     2
ntl        2018-10-09   3
web        2018-10-12   3
web        2018-10-11   4
mob                     3

I want to update null values in 'date' for 'mob' platform by matching the 'id' column from platform 'web'. The result should look like this:

platform   date         id
---------------------------
web        2018-10-10   1
mob        2018-10-10   1
mob        2018-10-10   1
web        2018-10-15   2
mob        2018-10-15   2
ntl        2018-10-09   3
web        2018-10-12   3
web        2018-10-11   4
mob        2018-10-12   3

Will really appreciate your help!

Answer 1

You can use an updatable CTE:

with toupdate as (
      select t.*, max(date) over (partition by id) as max_date
      from t
     )
update toupdate
    set date = max_date
    where date is null;

Answer 2

A correlated subquery should work

declare @table table (platform char(3), [date] date, id int)
insert into @table
values
('web','2018-10-10',1),
('mob',null,1),
('mob',null,1),
('web','2018-10-15',2),
('mob',null,2),
('ntl','2018-10-09',3),
('web','2018-10-12',3),
('web','2018-10-11',4),
('mob',null,3)

update @table
set date = (select max(date) from @table t2 where t2.id = [@table].id)
where [@table].date is null  

select * from @table

Answer 3

I would not recommend naming a column date as it's a reserved word, but this will give you the desired result assuming there is only 1 web entry per id.

UPDATE a
SET [date] = b.[date]
FROM MyTable a
INNER JOIN MyTable b ON a.id = b.id and b.platform = 'web'
WHERE a.platform = 'mob' AND a.[date] IS NULL

Answer 4

You can do like

UPDATE T
SET [Date] = D
FROM
(
    SELECT ID, MAX([Date]) AS D
    FROM T
    WHERE [Date] IS NOT NULL
    GROUP BY ID
) TT INNER JOIN T ON T.ID = TT.ID
WHERE T.[Date] IS NULL;

db<>fiddle demo

Answer 5

update a
SET a.date = b.date
from #test AS a
INNER JOIN (SELECT * FROM #test WHERE platform = 'web') as b on a.id = b.id
WHERE a.date is null

Update the tablename #test as needed.