Flatten variable length DB rows into a single column - sql

Question

I've searched and can't find a solution to this that exactly fits my needs, nor can I find one that I can modify. I have a database table, for simplicity we'll say it has three columns (packageID, carrier, and sequence). For any package there can be one or more carriers that have handled the package. I can do a query like
SELECT packageID, carrier
FROM packageFlow
ORDER BY sequence
to get a list of all the people that have handled packages that looks like:
packageID, carrier
1, Bob
1, Jim
1, Sally
1, Ron
2, Reggie
2, Mary
2, Bruce
What I need though is to get the results into rows that look like:
packageID|carrier1|carrier2|carrier3|carrier4
1 |Bob |Jim |Sally |Ron
2 |Reggie |Mary |Bruce
Pivot doesn't seem to do what I need since I'm not aggregating anything and I can't get a CTE to work correctly either. I'd appreciate any nudges in the right direction.

Answer 1

This data transformation is a PIVOT. Starting in SQL Server 2005, there is a function that will convert the rows into columns.
If you have a known number of values, then you can hard-code your query:
select *
from
(
select packageid, carrier,
'Carrier_'+cast(row_number() over(partition by packageid order by packageid) as varchar(10)) col
from packageflow
) src
pivot
(
max(carrier)
for col in (Carrier_1, Carrier_2, Carrier_3, Carrier_4)
) piv
See SQL Fiddle with Demo.
If you have an unknown number of Carrier values that you want to turn into columns, then you can use dynamic sql:
DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
select #cols = STUFF((SELECT distinct ',' + QUOTENAME(t.col)
from
(
select 'Carrier_'+cast(row_number() over(partition by packageid order by packageid) as varchar(10)) col
from packageFlow
) t
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set #query = 'SELECT packageid,' + #cols + ' from
(
select packageid, carrier,
''Carrier_''+cast(row_number() over(partition by packageid order by packageid) as varchar(10)) col
from packageflow
) x
pivot
(
max(carrier)
for col in (' + #cols + ')
) p '
execute(#query)
See SQL Fiddle with Demo.
Note: you will replace the order by packageid with order by sequence
The result of both queries is:
| PACKAGEID | CARRIER_1 | CARRIER_2 | CARRIER_3 | CARRIER_4 |
-------------------------------------------------------------
| 1 | Bob | Jim | Sally | Ron |
| 2 | Reggie | Mary | Bruce | (null) |