Flattening 1-to-many relationship - sql

Question

My current schema looks like this:
PersonType (PersonTypeID, Name, Description)
Person (PersonID,PersonTypeID, FirstName, ... )
PersonDynamicField (PersonDynamicFieldID, PersonTypeID, Name, Description, DataType, DefaultValue, ...)
PersonDynamicFieldValue (PersonDynamicFieldValueID, PersonDynamicFieldID, PersonID, Value, ...)
That is, a person is of a certain type. For example, Customer. For each PersonType, there can dynamically be added additional fields to store about a PersonType. For a Customer, we might want to add fields to PersonDynamicField such as LikesChocolate, FavoriteColor, HappinessScale, etc. The value for those fields would then be stored in PersonDynamicFieldValue.
I hope my writing makes sense.
What I would like to do, is a query that can flatten this structure and return a result looking like this:
PersonID, PersonTypeID, FirstName, LikesChocolate, FavoriteColor, HappinessScale
1, 2, Robert, 1, Green, 9
2, 2, John, 0, Orange, 5
...
I'm kind of stuck and don't really know where to even start.
Can you help?

Answer 1

In order to get the result that you want there are several ways that you can convert the rows of data into columns.
Starting in SQL Server 2005, you can use the PIVOT function. The basic structure of the code will be:
SELECT personid, persontypeid, firstname,[FavoriteColor],[HappinessScale],[LikesChocolate]
from
(
select p.personid, p.persontypeid, p.firstname, f.name fields, v.value
from person p
inner join persontype pt
on p.persontypeid = pt.persontypeid
left join PersonDynamicField f
on p.PersonTypeID = f.PersonTypeID
left join PersonDynamicFieldValue v
on f.PersonDynamicFieldID = v.PersonDynamicFieldID
and p.personid = v.personid
) x
pivot
(
max(value)
for fields in ([FavoriteColor],[HappinessScale],[LikesChocolate])
) p;
See SQL Fiddle with Demo. The one issue that you are going to have with PIVOT is that it requires that the values being converted to columns are known at run-time. For your situation, this seems impossible since the values can change. As a result, you will have to use dynamic SQL to get the result:
DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
select #cols = STUFF((SELECT distinct ',' + QUOTENAME(Name)
from PersonDynamicField
where PersonTypeID = 2
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set #query = 'SELECT personid, persontypeid, firstname,' + #cols + '
from
(
select p.personid,
p.persontypeid,
p.firstname,
f.name fields,
v.value
from person p
inner join persontype pt
on p.persontypeid = pt.persontypeid
left join PersonDynamicField f
on p.PersonTypeID = f.PersonTypeID
left join PersonDynamicFieldValue v
on f.PersonDynamicFieldID = v.PersonDynamicFieldID
and p.personid = v.personid
) x
pivot
(
max(value)
for fields in (' + #cols + ')
) p '
execute(#query);
See SQL Fiddle with Demo. These will give a result:
| PERSONID | PERSONTYPEID | FIRSTNAME | FAVORITECOLOR | HAPPINESSSCALE | LIKESCHOCOLATE |
-----------------------------------------------------------------------------------------
| 1 | 2 | Robert | Green | 9 | 1 |
| 2 | 2 | John | Orange | 5 | 0 |

Answer 2

What you want is commonly called a pivot and SQL Server has an operation that may help. Take a look at this article on MSDN for examples: http://msdn.microsoft.com/en-us/library/ms177410(v=sql.105).aspx