Question

I use a Linq-2-Sql at the moment in order to perform a fairly complicated query. It is however very inefficient and results in multiple round trips being made to the server.

Using some joins I have managed to get the data into a format like this:

Sensor -- Timestamp        -- Value
A      -- 12/02/2013 09:00 -- 10.4
A      -- 12/02/2013 10:00 -- 10.3
A      -- 12/02/2013 11:00 -- 10.1
B      -- 12/02/2013 09:00 -- 15.3
B      -- 12/02/2013 10:00 -- 16.4
B      -- 12/02/2013 11:00 -- 15.4

I desire this in an output like

TimeStamp        -- SensorA -- SensorB
12/02/2013 09:00 -- 10.4    -- 15.3
12/02/2013 10:00 -- 10.3    -- 16.4
12/02/2013 11:00 -- 10.1    -- 15.4

I am guessing I need a dynamic pivot? Can anybody help point me in the right direction?

EDIT – Obviously there are more than the 2 sensors…ideally I need this to be able to scale out in terms of columns. I don’t even know if SQL Server can do this so any help is appreciated.

Answer 1

You will want to use the PIVOT function to transform the data.

If you have a limited number of Sensor values, then you can hard-code the query:

select *
from
(
  select sensor, timestamp, value
  from yourtable
) src
pivot
(
  max(value)
  for sensor in (A, B)
) piv

See SQL Fiddle with Demo

But if the values are unknown then you will want to implement dynamic SQL:

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

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

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

execute(@query)

See SQL Fiddle with Demo

Both will give the result:

|           TIMESTAMP |    A |    B |
-------------------------------------
| 2013-12-02 09:00:00 | 10.4 | 15.3 |
| 2013-12-02 10:00:00 | 10.3 | 16.4 |
| 2013-12-02 11:00:00 | 10.1 | 15.4 |

Note, you will replace the yourtable with your current query.

Edit #1, if you want to filter by date, you could use the following dynamic sql:

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX),
    @startdate datetime,
    @enddate datetime

set @startdate = '2013-12-01'
set @enddate = '2013-12-03'

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

set @query = 'SELECT timestamp, ' + @cols + ' from 
             (
                select sensor, timestamp, value
                from yourtable
                where timestamp >= '''+convert(varchar(10), @startdate, 120)+'''
                  and timestamp <= '''+convert(varchar(10), @enddate, 120)+'''
            ) x
            pivot 
            (
                max(value)
                for sensor in (' + @cols + ')
            ) p '


execute(@query)

See SQL Fiddle with Demo