Selecting Values From a table as Column Headers

Question

I have a table 'propertyvalues' as follows:

ID  FileID  Property  Value

 1   x       Name      1.pdf

 2   x       Size      12567

 3   x       Type      application/pdf

 4   y       Name      2.pdf

 5   y       Size      23576

 6   y       Type      application/pdf

......

and so on

How to write a SQL query on the table above to fetch a result like below

 
FileID  Name     Size      Type

  x     1.pdf    12567     application/pdf

  y     2.pdf    23576     application/pdf

Answer 1

You did not specify RDBMS, if you know the number of columns to transform then you can hard-code the values:

select FileId,
  max(case when property = 'Name' then value end) Name,
  max(case when property = 'Size' then value end) Size,
  max(case when property = 'Type' then value end) Type
from yourtable
group by FileId

This is basically a PIVOT function, some RDBMS will have a PIVOT, if you do then you can use the following, PIVOT is available in SQL Server, Oracle:

select *
from 
(
  select FileId, Property, Value
  from yourTable
) x
pivot
(
  max(value)
  for property in ([Name], [Size], [Type])
) p

If you have an unknown number of columns to transform, then you can use a dynamic PIVOT. This gets the list of columns to transform at run-time:

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT distinct ',' + QUOTENAME(property) 
                    from yourtable
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT ' + @cols + ' from 
             (
                select FileId, Property, Value
                from yourtable
            ) x
            pivot 
            (
                max(value)
                for Property in (' + @cols + ')
            ) p '

execute(@query)