Simple SQL pivot | 易学教程

问题

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.

回答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

来源：https://stackoverflow.com/questions/63068935/simple-sql-pivot

标签

sql

sql-server