SQL Pivot 2nd step -- Combine rows / Remove nulls - sql

Question

I am trying to pivot data. We start with two columns: ListNum and Value
The row ?integrity? doesn't matter, I just want all the values to collapse upwards and remove the nulls.
In this case ListNum is like an enum, the values are limited to List1, List2, or List3. Notice that they are not in order (1,3,3,1,2 rather than 1,2,3,1,2,3 etc.).
It would be nice to have a solution that uses standard sql so it would work across many databases.
Starting Point:
+---------+------------+
| ListNum | Value |
+---------+------------+
| List1 | A |
| List3 | 123 |
| List3 | CDE |
| List1 | Somestring |
| List2 | randString |
+---------+------------+
I was able to separate the Lists into columns with:
select
case when ListNum = "List1" then Value end as List1,
case when ListNum = "List2" then Value end as List2,
case when ListNum = "List3" then Value end as List3
from Table;
Midpoint:
+------------+------------+-------+
| List1 | List2 | List3 |
+------------+------------+-------+
| A | NULL | NULL |
| NULL | NULL | 123 |
| NULL | NULL | CDE |
| Somestring | NULL | NULL |
| NULL | randString | NULL |
+------------+------------+-------+
but now I need to collapse upwards/remove the nulls to get -
Desired Output:
+------------+------------+-------+
| List1 | List2 | List3 |
+------------+------------+-------+
| A | randString | 123 |
| Somestring | NULL | CDE |
+------------+------------+-------+

Answer 1

Aren't you missing some kind of grouping criterion? How do you determine, that A belongs to 123 and not to CDE? And why is randString in the first line and not in the second?
This is easy, with such a grouping key:
DECLARE #tbl TABLE(GroupingKey INT, ListNum VARCHAR(100),[Value] VARCHAR(100));
INSERT INTO #tbl VALUES
(1,'List1','A')
,(1,'List3','123')
,(2,'List3','CDE')
,(2,'List1','Somestring')
,(1,'List2','randString');
SELECT p.*
FROM #tbl
PIVOT
(
MAX([Value]) FOR ListNum IN(List1,List2,List3)
) p;
But with your data this seems rather random...
UPDATE: A random approach...
The following approach will sort the values into their columns rather randomly:
DECLARE #tbl TABLE(ListNum VARCHAR(100),[Value] VARCHAR(100));
INSERT INTO #tbl VALUES
('List1','A')
,('List3','123')
,('List3','CDE')
,('List1','Somestring')
,('List2','randString');
--This will use three independant, but numbered sets and join them:
WITH All1 AS (SELECT [Value],ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS RandomNumber FROM #tbl WHERE ListNum='List1')
,All2 AS (SELECT [Value],ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS RandomNumber FROM #tbl WHERE ListNum='List2')
,All3 AS (SELECT [Value],ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS RandomNumber FROM #tbl WHERE ListNum='List3')
SELECT All1.[Value] AS List1
,All2.[Value] AS List2
,All3.[Value] AS List3
FROM All1
FULL OUTER JOIN All2 ON All1.RandomNumber=All2.RandomNumber
FULL OUTER JOIN All3 ON All1.RandomNumber=All3.RandomNumber ;
Hint: There is no implicit sort order in your table!
From your comment:
It’s simply the index / instance number. randString is the first non-null row.
Without a specific ORDER BY the same SELECT may return your data in any random order. So there is no first non-null row, at least not in the meaning of first comes before second...

Answer 2

Something with recursive CTE may work:
DECLARE #tbl TABLE( ListNum VARCHAR(100),[Value] VARCHAR(100));
INSERT INTO #tbl VALUES
( 'List1','A')
,( 'List3','123')
,( 'List3','CDE')
,( 'List1','Somestring')
,( 'List2','randString');
DECLARE #mmax int;
SELECT #mmax = cnt from (SELECT TOP 1 count(*) cnt from #tbl group by ListNum ORDER BY count(*) DESC) t;
With rec AS (
SELECT 1 AS num
UNION ALL
SELECT num+1 FROM rec WHERE num+1<=#mmax
)
SELECT t1.List1, t2.List2, t3.List3 FROM rec
FULL JOIN (
select Value as List1, row_number() over(order by ListNum) rn from #tbl where ListNum = 'List1'
) t1
ON rec.num = t1.rn
FULL JOIN
(
select Value as List2, row_number() over(order by ListNum) rn from #tbl where ListNum = 'List2'
) t2
ON rec.num = t2.rn
FULL JOIN
(
select Value as List3, row_number() over(order by ListNum) rn from #tbl where ListNum = 'List3'
) t3
ON rec.num = t3.rn;
DEMO

Answer 3

Ordinarily you'd use an aggregating operation such as MAX because it hides nulls (a null can never be the max unless there is no other valid value in the group).. but your query is a bit odd because you don't seem to have a solid pivot anchor, and you're allowing any of your data to become associated with anything else. In the real world this probably wouldn't happen because it isn't particularly useful
Better example data:
Person, Attribute, Value
1, Name, John
1, Age, 10
2, Name, Sarah
3 Age, 39
Pivoting query:
SELECT
Person,
MAX(case when attribute = 'name' then value end) as name,
MAX(case when attribute = 'age' then value end) as age
FROM
data
GROUP BY person
Result:
Person, Name, Age
1, John, 10
2, Sarah, NULL
3, NULL, 39