What would be a good way to pivot this data? - sql

Question

I have data in the following granularity:
CityID | Name | Post_Science | Post_Science | Post_Reading | Pre_Reading | Post_Writing | Pre_Writing
123 | Bob | 2.0 | 1.0 | 2.0 | 4.0 | 1.0 | 1.0
I'll be calling those <Post/Pre>_XXXXXX columns as Labels. Basically, these column names without the 'Pre' or 'Post' text map to a Label in another table.
I want to pivot the data in a way so that the pre and post values of the same Label are in the same row, for each group of CityID, Name, Label. So it would look like this:
CityID | Name | Pre Category | Post Category | Label
123 | Bob | 1.0 | 2.0 | Science
123 | Bob | 4.0 | 2.0 | Reading
123 | Bob | 1.0 | 1.0 | Writing
The Label comes from a separate table via a join. Hopefully that doesn't confuse anyone. If it does, ignore the column for now.
So there are much more of these categories - Science, Reading, and Writing are just a few I picked for example.
I've thought of two options to get the data in this format:
Unpivot all the data into a long list of all the values at a group of CityID, Name, Label. Then parse the Label name and pivot back into the pre and post values of one category into 1 row
Do a bunch of Unions. So select all Science in one select statement, all the Reading in another select statement and union them. There are about 50 pairings, so 50 union statements
I'm imagining the first option is cleaner than the latter. Any other options though?

Answer 1

This is unpivoting and I strongly recommend apply:
select t.CityId, t.Name, v.*
from t cross apply
(values (t.Post_Science, t.Pre_Science, 'Science'),
(t.Post_Reading, t.Pre_Reading, 'Reading'),
(t.Post_Writing, t.Pre_Writing, 'Writing')
) v(postcategory, precategory, label) ;
UNPIVOT is very particular syntax to do one thing. APPLY introduces lateral joins, which are very powerful for this and many other purposes.

Answer 2

Clearly Gordon's solution would be more performant, but if you have MANY or VARIABLE COLUMNS, here is an option that will dynamically UNPIVOT your data without actually using DYNAMIC SQL
Example
Select A.CityID
,A.Name
,PreCat = max(case when Item Like 'Pre%' then Value end)
,PostCat = max(case when Item Like 'Post%' then Value end)
,Label = substring(Item,charindex('_',Item+'_')+1,50)
From YourTable A
Cross Apply ( values (cast((Select A.* for XML RAW) as xml))) B(XMLData)
Cross Apply (
Select Item = xAttr.value('local-name(.)', 'varchar(100)')
,Value = xAttr.value('.','varchar(max)')
From XMLData.nodes('//#*') xNode(xAttr)
Where xAttr.value('local-name(.)','varchar(100)') not in ('CityId','Name','Other-Columns','To-Exclude')
) C
Group By A.CityID
,A.Name
,substring(Item,charindex('_',Item+'_')+1,50)
Returns
CityID Name PreCat PostCat Label
123 Bob 4.0 2.0 Reading
123 Bob 1.0 2.0 Science
123 Bob 1.0 1.0 Writing