Managed to get it to do what I wanted with the below...not sure its 'pure' but it works. The compare functions seems to expect a table?
!path lib/*.jar
!contents -R -g -p -f -h
!|dbfit.SqlServerTest|
|Set parameter|expectedValue1|0|
!|ConnectUsingFile|FitNesseRoot\files\Connections\U_ODS.txt|
|Query|SELECT SUM(Total) AS TotalRecords_ODS FROM (SELECT COUNT(*) AS Total FROM TABLEA UNION SELECT COUNT(*) AS Total FROM TABLEB) records|
|TotalRecords_ODS?|
|>>expectedValue1|
!|ConnectUsingFile|FitNesseRoot\files\Connections\U_DW.txt|
|Query| select count(*) - @expectedValue1 as expectedDifference from TABLEC|
|expectedDifference|
|0|