Check for null and replace the most recent not null value - sql

Question

The Query needs to Check for null value and replace with the most recent not null value.
For example consider the source data provided below .
Query needs to check the null value and found it in second row, now it should replace the null value with recent not null data and the data is"colmin".
Again the Query need to check the null value and Now it found it in third row and then it will update with recent not null data and the data is again "colmin".
Thanks in advance !
Input
source data
Colmin
NULL
NULL
NULL
NULL
columnxy
column99
NULL
NULL
money
NULL
NULL
NULL
start end
NULL
so the output should look like..
output
Ouput data
Colmin
Colmin
Colmin
Colmin
Colmin
Colmin
columnxy
column99
column99
column99
money
money
money
money
start end
start end

Answer 1

What is "most recent" ? I hope you've got some field to sort on. Row number MUST NOT ALWAYS be in right order! Though I'd use orderField which is will be used to determine the row order.
UPDATE myTable
SET a = (
SELECT a
FROM myTable
WHERE a IS NOT NULL
AND orderField > (
SELECT orderField
FROM myTable
WHERE a IS NULL
ORDER BY orderField
LIMIT 1
)
ORDER BY orderField
LIMIT 1
)
WHERE a IS NULL
ORDER BY orderField
something like this should do it ... i hope. it's untested.
what it does:
Find "orderField" for first row with a = null
Find first a value (!= null) AFTER orderField from 1.
Update a value from 1. with value from 2.
It should also work more easy:
UPDATE myTable t1
SET t1.a = (
SELECT t2.a
FROM myTable t2
WHERE t2.a IS NOT NULL
AND t2.orderField > t1.orderField
ORDER BY t2.orderField
LIMIT 1
)
WHERE t1.a IS NULL

Answer 2

Try this:
declare #input table (id bigint not null identity(1,1), OutputData nvarchar(16) null)
insert #input select 'Colmin'
insert #input select NULL
insert #input select NULL
insert #input select NULL
insert #input select NULL
insert #input select 'columnxy'
insert #input select 'column99'
insert #input select NULL
insert #input select NULL
insert #input select 'money'
insert #input select NULL
insert #input select NULL
insert #input select NULL
insert #input select 'start end'
insert #input select NULL
--where a's value is null, replace with a non-null from b
select coalesce(a.OutputData, b.OutputData)
--return all rows from input as a
from #input a
--return all non-null values as b
left outer join
(
select id, OutputData
from #input
where OutputData is not null
) b
--the non-null value should come before the null value (< or <= works equally here)
on b.id <= a.id
--and there shouldn't be a non-null value between b and a's records (i.e. b is the most recent before a)
and not exists (
select top 1 1
from #input c
where c.id between b.id and a.id
and c.id <> b.id
and c.OutputData is not null
)
--display the output in the required order
order by a.id

Answer 3

If your table is a single column, then you will need to either run multiple queries in a loop or use a cursor to iterate through the table, one row at a time. (not efficient.)
If you have some kind of ID column, then you can use a correlated subquery to find the first non-null value. Something like...
Update A
Set TextCol = (SELECT Top 1 TextCol From MyTable B Where B.TextCol IS NOT NULL AND B.IDCol < A.IDCol ORDER BY IDCol DESC)
FROM MyTable A
WHERE TextCol IS NULL