sql server merge rows and get latest value - sql

Question

I have a table like this,
Id A B C D touchedwhen
1 NULL yes NULL yes 2015-02-26 14:10:01.870
1 NULL NULL no no 2015-02-26 14:10:40.370
and need to merge them in to one row like this,
Id A B C D touchedwhen
1 NULL yes no no 2015-02-26 14:10:40.370.
Note : if value is present in both rows take the latest one by date..
Tried this query:
select id,
max(a),
max(b),
max(c),
max(d), -- data in both rows hence take the latest
max(touchedwhen)
from
[dbo].[Table_1]
group by id;

Answer 1

If you were just looking for the last value per id, you could use:
last_value(a) over(
partition by id
order by touchedwhen desc
rows between unbounded preceding and unbounded following) as a
But you're looking for the last value per id that is not null. The only thing I can come up with is a four-part join, with each subquery calculating the latest non-null value for a, b, c, d:
select ids.id
, a.a
, b.b
, c.c
, d.d
, ids.tw
from (
select id
, max(touchedwhen) as tw
from YourTable
group by
id
) ids
left join
(
select row_number() over (
partition by id
order by touchedwhen desc) rn
, a
, id
from YourTable
where a is not null
) a
on a.id = ids.id
and a.rn = 1
left join
(
select row_number() over (
partition by id
order by touchedwhen desc) rn
, b
, id
from YourTable
where b is not null
) b
on b.id = ids.id
and b.rn = 1
left join
(
select row_number() over (
partition by id
order by touchedwhen desc) rn
, c
, id
from YourTable
where c is not null
) c
on c.id = ids.id
and c.rn = 1
left join
(
select row_number() over (
partition by id
order by touchedwhen desc) rn
, d
, id
from YourTable
where d is not null
) d
on d.id = ids.id
and d.rn = 1

Answer 2

-- Get all latest values in one row for each primary key
WITH CTE(row_num, Id, A, B, C, D, touchedwhen) AS (
SELECT ROW_NUMBER() OVER(PARTITION BY Id ORDER BY touchedwhen DESC), Id, A, B, C, D FROM Table_1)
UPDATE CTE SET
A = (SELECT TOP 1 t.A FROM Table_1 t WHERE t.A IS NOT NULL AND t.Id = CTE.Id ORDER BY t.touchedwhen DESC),
B = (SELECT TOP 1 t.B FROM Table_1 t WHERE t.B IS NOT NULL AND t.Id = CTE.Id ORDER BY t.touchedwhen DESC),
C = (SELECT TOP 1 t.C FROM Table_1 t WHERE t.C IS NOT NULL AND t.Id = CTE.Id ORDER BY t.touchedwhen DESC),
D = (SELECT TOP 1 t.D FROM Table_1 t WHERE t.D IS NOT NULL AND t.Id = CTE.Id ORDER BY t.touchedwhen DESC)
WHERE row_num = 1
-- Delete extra rows per primary key after copying latest values to one row
WITH CTE(row_num) AS (
SELECT ROW_NUMBER() OVER(PARTITION BY Id ORDER BY touchedwhen DESC) FROM Table_1)
DELETE FROM CTE WHERE row_num > 1