Get the latest value for each column - sql

Question

Suppose I have the following "values" table in my SQL Server (2012) DB:
Table1:
Id Col1 Col2 Col3 Col4
And I want to create a second "override" table that will store values to override the original values in case a user needs to do so. So, given the table above, the override table would look as follows:
Overrides:
FK_Id Col1 Col2 Col3 Col4 When_Inserted
Where Overrides.FK_Id references Table1.Id as a foreign key.
So, for example, suppose my Overrides table had the following rows within it with overrides for a row in Table1 with Id=1:
FK_Id: Col1: Col2: Col3: Col4: When_Inserted:
1 Val1_1 Val2_1 Expected_Val3 NULL 1-Jan
1 NULL Val2_2 NULL NULL 2-Jan
1 NULL Expected_Val2 NULL NULL 3-Jan
1 Expected_Val1 NULL NULL NULL 4-Jan
Then, based upon the When_Inserted column - Wanting the latest inserts to take precedence, I'd want the overrides to be as follows:
FK_Id: Col1: Col2: Col3: Col4:
1 Expected_Val1 Expected_Val2 Expected_Val3 NULL
I'm trying to think of a smart way to create this SQL and am coming up with a fairly ugly solution along the lines of:
SELECT
FK_Id
,(
SELECT TOP 1
Col1
FROM
Overrides O1
WHERE
Col1 IS NOT NULL
AND O1.FK_Id = O.FK_Id
ORDER BY
O1.When_Inserted DESC
) Col1
.... <same for each of the other columns> ....
FROM
Overrides O
GROUP BY
FK_Id
I'm sure there has to be a better way that is cleaner and substantially more efficient.

Answer 1

using a common table expression with row_number() (latest first), cross apply() to unpivot your columns, filter for the latest of each column (rn = 1), and finally pivot() back to the same form:
;with cte as (
select o.fk_id, v.Col, v.Value, o.When_Inserted
, rn = row_number() over (partition by o.fk_id, v.col order by o.when_inserted desc)
from overrides o
cross apply (values('Col1',Col1),('Col2',Col2),('Col3',Col3),('Col4',Col4)
) v (Col,Value)
where v.value is not null
)
select fk_id, col1, col2, col3, col4
from (
select fk_id, col, value
from cte
where rn = 1
) s
pivot (max(Value) for Col in (col1,col2,col3,col4)) p
rextester demo: http://rextester.com/KGM96394
returns:
+-------+---------------+---------------+---------------+------+
| fk_id | col1 | col2 | col3 | col4 |
+-------+---------------+---------------+---------------+------+
| 1 | Expected_Val1 | Expected_Val2 | Expected_Val3 | NULL |
+-------+---------------+---------------+---------------+------+
dbfiddle.uk demo comparison of 3 methods
Looking at the io stats for the sample:
unpivot/pivot version:
Table 'Worktable'. Scan count 0, logical reads 0
Table 'overrides'. Scan count 1, logical reads 1
first_value over() version:
Table 'Worktable'. Scan count 20, logical reads 100
Table 'overrides'. Scan count 1, logical reads 1
select top 1 subquery version:
Table 'overrides'. Scan count 5, logical reads 5
Table 'Worktable'. Scan count 0, logical reads 0

Answer 2

You can use first_value():
select distinct fkid,
first_value(col1) over (partition by fkid
order by (case when col1 is not null then 1 else 2 end),
when_inserted desc
) as col1,
first_value(col2) over (partition by fkid
order by (case when col2 is not null then 1 else 2 end),
when_inserted desc
) as col2,
. . .
from t;
The select distinct is because SQL Server does not have the equivalent functionality as an aggregation function.

Answer 3

See my solution is quite different.
IMHO, my script performance will be better provided it give correct output across all sample data.
I have use auto generated id in my script,but in case if you don't have identity id then you can use ROW_NUMBER . and my script is very easy to understand .
declare #t table(id int identity(1,1),FK_Id int,Col1 varchar(50),Col2 varchar(50)
,Col3 varchar(50),Col4 varchar(50),When_Inserted date)
insert into #t VALUES
(1 ,'Val1_1' ,'Val2_1' ,'Expected_Val3', NULL , '2017-01-1')
,(1 ,NULL ,'Val2_2' , NULL , NULL, '2017-01-2')
,(1 ,NULL ,'Expected_Val2', NULL , NULL, '2017-01-3')
,(1 ,'Expected_Val1' , NULL , NULL , NULL, '2017-01-4')
;
WITH CTE
AS (
SELECT *
,CASE
WHEN col1 IS NULL
THEN NULL
ELSE CONCAT (
cast(id AS VARCHAR(10))
,'_'
,col1
)
END col1Code
,CASE
WHEN col2 IS NULL
THEN NULL
ELSE CONCAT (
cast(id AS VARCHAR(10))
,'_'
,col2
)
END col2Code
,CASE
WHEN col3 IS NULL
THEN NULL
ELSE CONCAT (
cast(id AS VARCHAR(10))
,'_'
,col3
)
END col3Code
,CASE
WHEN col4 IS NULL
THEN NULL
ELSE CONCAT (
cast(id AS VARCHAR(10))
,'_'
,col4
)
END col4Code
FROM #t
)
,CTE1
AS (
SELECT FK_Id
,max(col1Code) col1Code
,max(col2Code) col2Code
,max(col3Code) col3Code
,max(col4Code) col4Code
FROM cte
GROUP BY FK_Id
)
SELECT FK_Id
,SUBSTRING(col1Code, charindex('_', col1Code) + 1, len(col1Code)) col1Code
,SUBSTRING(col2Code, charindex('_', col2Code) + 1, len(col2Code)) col2Code
,SUBSTRING(col3Code, charindex('_', col3Code) + 1, len(col2Code)) col3Code
,SUBSTRING(col4Code, charindex('_', col4Code) + 1, len(col4Code)) col4Code
FROM cte1 c1