SQL Server: Order cols in a row - sql

Question

I want to swap the value of columns to have them sorted.
Table:
pid | category1 | category2 | category3
----+-----------+-----------+----------
1 | a | b |
2 | b | a |
3 | a | c | b
Result should be:
pid | category1 | category2 | category3
----+-----------+-----------+----------
1 | a | b |
2 | a | b |
3 | a | b | c
My approach was to select the columns to rows, order them in groups an return the new columns:
pid | category
----+---------
1 | a
1 | b
2 | b
2 | a
3 | a
3 | c
3 | b
I found the pivot function, but didn't really understand how to use it in this context.

Answer 1

Sean Lange is right about how you should correct your database schema.
Here is something to get you your results until then:
Using cross apply(values ...) to unpivot your data along with row_number() to reorder your category inside a common table expression; then repivoting that data with conditional aggregation:
;with cte as (
select
t.pid
, u.category
, rn = row_number() over (partition by t.pid order by u.category)
from t
cross apply (values (category1),(category2),(category3)) u(category)
where nullif(u.category,'') is not null
)
select
pid
, category1 = max(case when rn = 1 then category end)
, category2 = max(case when rn = 2 then category end)
, category3 = max(case when rn = 3 then category end)
from cte
group by pid
rextester demo: http://rextester.com/GIG22558
returns:
+-----+-----------+-----------+-----------+
| pid | category1 | category2 | category3 |
+-----+-----------+-----------+-----------+
| 1 | a | b | NULL |
| 2 | a | b | NULL |
| 3 | a | b | c |
+-----+-----------+-----------+-----------+

Answer 2

you can pivot as below
select * from (
select *, RowN = row_number() over(partition by pid order by Category) from Categories
) a
pivot (max(category) for RowN in ([1],[2],[3])) p
Output as below:
+-----+-----------+-----------+-----------+
| Pid | Category1 | Category2 | Category3 |
+-----+-----------+-----------+-----------+
| 1 | a | b | NULL |
| 2 | a | b | NULL |
| 3 | a | b | c |
+-----+-----------+-----------+-----------+
For dynamic list of columns you can use as below:
declare #cols1 varchar(max)
declare #cols2 varchar(max)
declare #query nvarchar(max)
--Row Numbers with tally
;with c1 as (
select * from ( values (1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) v(n) )
,c2 as (select n1.* from c1 n1, c1 n2, c1 n3, c1 n4)
,RowNumbers as (
select top (select max(cnt) from (select cnt = count(*) from Categories group by pid ) a) convert(varchar(6), row_number() over (order by (select null))) as RowN
from c2 n1, c2 n2
)
select #cols1 = stuff((select ','+QuoteName(RowN) from RowNumbers group by RowN for xml path('')),1,1,''),
#cols2 = stuff((select ',' + QuoteName(RowN) + ' as '+ QuoteName(concat('Category' , RowN)) from RowNumbers group by RowN for xml path('')),1,1,'')
select #cols1, #cols2
Set #query = ' Select Pid, '+ #cols2 +' from ( '
Set #query += ' select *, RowN = row_number() over(partition by pid order by Category) from Categories) a '
Set #query += ' pivot (max(category) for RowN in (' + #cols1 + ')) p'
--select #query
exec sp_executesql #query