In SQL Server, Is it possible to pivot rows without aggregation? - tsql

Question

Here is an example: Assuming that each command has a limited number of parameters, I would like to represent the parameters and their values as named_indexed columns.
--drop table TCommand
--drop table TParam
Create Table TCommand (
CommandID INT,
CommandName NCHAR(20),
Description NVARCHAR(100)
);
Create Table TParam (
CommandID INT,
ParamName NCHAR(20),
ParamValue NCHAR(50)
);
insert into TCommand Values(1, 'C1', 'Desc for command C1')
insert into TCommand Values(2, 'C2', 'Desc for command C2')
insert into TCommand Values(3, 'C3', 'Desc for command C3')
insert into TParam Values (1, 'Pa', 'xa1')
insert into TParam Values (1, 'Pb', 'yb1')
insert into TParam Values (1, 'Pc', 'zc1')
insert into TParam Values (2, 'Px', 'xa2')
insert into TParam Values (2, 'Py', 'yb2')
insert into TParam Values (3, 'Pt', 'xa3')
insert into TParam Values (3, 'Pu', 'yb3')
select tc.*, tp.ParamName, tp.ParamValue
from TCommand tc
join TParam tp on tp.CommandID=tc.CommandID
order by tc.CommandName, tp.ParamName
Results:
CommandID CommandName Description ParamName ParamValue
----------- ----------- -------------------- --------- ----------
1 C1 Desc for command C1 Pa xa1
1 C1 Desc for command C1 Pb yb1
1 C1 Desc for command C1 Pc zc1
2 C2 Desc for command C2 Px xa2
2 C2 Desc for command C2 Py yb2
3 C3 Desc for command C3 Pt xa3
3 C3 Desc for command C3 Pu yb3
Here is the format I would like to obtain.
CommandID CommandName Description ParamName_1 ParamValue_1 ParamName_2 ParamValue_2 ParamName_3 ParamValue_3
----------- ----------- -------------------- ----------- ------------ ----------- ------------ ----------- ------------
1 C1 Desc for command C1 Pa xa1 Pb yb1 Pc zc1
2 C2 Desc for command C2 Px xa2 Py yb2 NULL NULL
3 C3 Desc for command C3 Pt xa3 Pu yb3 NULL NULL
What query should I write? Earlier attempts failed using PIVOT because of missing Aggregation function (which I thought I do not need).
Thanks in advance.

Answer 1

You need to create two pivot queries here: one for ParamName and one for ParamValue. In the q1 and q2 derived tables I have selected only the needed columns for each pivot (if I would have used just q in both queries, I would get extra rows with NULLs in the relevant columns).
To be able to join the two queries, you need a column representing the position of the parameter (named RowNum in the query below).
If there is a single value for each value of the pivot column, you can use an aggregator function such as MIN or MAX (which ignores the NULL values and keeps the single input value).
Therefore, you can use the following query:
;WITH q AS (
SELECT tp.CommandID,
ROW_NUMBER() OVER (PARTITION BY tp.CommandID ORDER BY tp.ParamName) AS RowNum,
tp.ParamName, tp.ParamValue
FROM dbo.TParam tp
)
SELECT tc.CommandID, tc.CommandName, tc.Description,
x1.ParamName_1, x2.ParamValue_2, x1.ParamName_2, x2.ParamValue_2, x1.ParamName_3, x2.ParamValue_3
FROM dbo.TCommand tc
LEFT JOIN (
SELECT p.CommandID, p.[1] AS ParamName_1, p.[2] AS ParamName_2, p.[3] AS ParamName_3
FROM (SELECT q.CommandID, q.RowNum, q.ParamName FROM q) q1
PIVOT (MAX(ParamName) FOR RowNum IN ([1],[2],[3])) p
) x1 ON x1.CommandID = tc.CommandID
LEFT JOIN (
SELECT p.CommandID, p.[1] AS ParamValue_1, p.[2] AS ParamValue_2, p.[3] AS ParamValue_3
FROM (SELECT q.CommandID, q.RowNum, q.ParamValue FROM q) q2
PIVOT (MAX(ParamValue) FOR RowNum IN ([1],[2],[3])) p
) x2 ON x2.CommandID = tc.CommandID