Simple SQL pivot - sql

Question

Googling SQL PIVOT brings up answers to more complex situations than I need with aggregations, and although I did find this simple SQL Pivot Query , it's pivoting on a single table, whereas I have two, it's doing a rank partition which I don't know is necessary, I can't actually get it to work, plus it's 5 years old and I'm hoping there's an easier way.
I am sure this is a duplicate question so if someone can find it then please do!
People table:
PersonID
========
1
2
3
Device table:
DeviceID | PersonID
===================
1111 1
2222 1
3333 1
123 2
456 2
9999 3
I do a join like this:
SELECT p.PersonID, d.DeviceID FROM People p
LEFT JOIN Device d on d.PersonID = p.PersonID
Which gives me:
PersonID | DeviceID
===================
1 1111
1 2222
1 3333
2 123
2 456
3 9999
I know what you're thinking, it's just the Device table, but this is a minimal version of the query and tables, there's much more going on in the real ones,
I want to be able to inject a join on the People table to the Device table and get three columns:
Must I use PIVOT to get the results like this? (there will always be a max of three devices per person)
PersonID | 1 | 2 | 3
===============================================
1 1111 2222 3333
2 123 456
3 9999
(Where the blanks would be NULL)
I'm trying:
SELECT PersonID, [1], [2], [3]
FROM (
SELECT p.PersonID, d.DeviceID FROM People p
LEFT JOIN Device d on d.PersonID = p.PersonID) AS r
PIVOT
(
MAX(DeviceID)
FOR DeviceID IN([1], [2], [3])
) AS p;
But it's giving me NULL for all three columns.

Answer 1

The value list defined in the pivot clause must contain actual values from your table. [1], [2], [3] are values from your PersonId, not for DeviceId. So the part for DeviceId in [1], [2], [3] is not producing any results, hence all the null values.
Here is my solution. I constructed a new key_ column to pivot around.
Sample data with added person names
declare #person table
(
personid int,
personname nvarchar(100)
);
insert into #person (personid, personname) values
(1, 'Ann'),
(2, 'Britt'),
(3, 'Cedric');
declare #device table
(
personid int,
deviceid int
);
insert into #device (personid, deviceid) values
(1, 1111),
(1, 2222),
(1, 3333),
(2, 123),
(2, 456),
(3, 9999);
Solution
Run the CTE part on its own to see the intermediate result table. The key_ column contains values like DEVICE_* which are the same values used in the for key_ in part of the pivot clause.
with base as
(
select p.personname,
d.deviceid,
'DEVICE_' + convert(char, ROW_NUMBER() over(partition by p.personname order by d.deviceid)) as 'key_'
from #person p
join #device d
on d.personid = p.personid
)
select piv.personname, piv.DEVICE_1, piv.DEVICE_2, piv.DEVICE_3
from base
pivot( max(deviceid) for key_ in ([DEVICE_1], [DEVICE_2], [DEVICE_3]) ) piv;
Result
The intermediate CTE result table
personname deviceid key_
---------- ----------- ----------
Ann 1111 DEVICE_1
Ann 2222 DEVICE_2
Ann 3333 DEVICE_3
Britt 123 DEVICE_1
Britt 456 DEVICE_2
Cedric 9999 DEVICE_1
The final result
personname DEVICE_1 DEVICE_2 DEVICE_3
---------- ----------- ----------- -----------
Ann 1111 2222 3333
Britt 123 456 NULL
Cedric 9999 NULL NULL