What are some good ways to transpose data in a SQL

Question

What are some good ways to transpose data in a SQL table from row-columns to column-rows?

Also allowing filters/where conditions to be applied on the initial query.

Using SQL Server 2008.

Existing table has following Columns: AID (nvarchar unique) ASID (nvarchar unique) Milestone (M1, M2, M3...M100) MilestoneDate (datetime)

Transposed data as follows: AID, ASID, M1 Date, M2 Date, M3 Date, M5 Date

Answer 1

If you are using SQL Server 2005+, then you have a few options to transpose the data. You can implement the PIVOT function similar to this:

select AID, ASID, 
  M1 as M1_Date, M2 as M2_Date, 
  M3 as M3_Date, M4 as M4_Date, M5 as M5_Date
from 
(
  select AID, ASID, Milestone,
    MilestoneDate
  from yourtable
  where AID = whatever -- other filters here
) src
pivot
(
  max(milestonedate)
  for milestone in (M1, M2, M3, M4, M5...)
) piv

Or you can use an aggregate function with a CASE statement:

select aid, 
  asid, 
  max(case when milestone = 'M1' then milestonedate else null end) M1_Date,
  max(case when milestone = 'M2' then milestonedate else null end) M2_Date,
  max(case when milestone = 'M3' then milestonedate else null end) M3_Date,
  max(case when milestone = 'M4' then milestonedate else null end) M4_Date
from yourtable
where AID = whatever -- other filters here 
group by aid, asid

The above two queries work great if you have a known number of milestone values. But if not, then you can implement dynamic sql to transpose the data. The dynamic version would be similar to this:

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

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

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

set @query = 'SELECT AID, ASID,' + @colNames + ' from 
             (
                select AID, ASID, Milestone,
                  MilestoneDate
                from yourtable
                where AID = whatever -- other filters here 
            ) x
            pivot 
            (
                max(MilestoneDate)
                for Milestone in (' + @cols + ')
            ) p '

execute(@query)

Answer 2

This is a fairly generic approach:

select 
aid, asid, 
max (case when milestone = 'M1' then milestonedate else null end) M1Date,
max (case when milestone = 'M2' then milestonedate else null end) M2Date,
max (case when milestone = 'M3' then milestonedate else null end) M3Date,
max (case when milestone = 'M5' then milestonedate else null end) M5Date
from
mytable
group by aid, asid