You don't have any variables in your code.

You can check the CREATE FUNCTION syntax in here http://msdn.microsoft.com/es-es/library/ms186755%28v=sql.90%29.aspx

CREATE FUNCTION dbo.Example_function_name

(

@Parameter1 int

)

RETURNS TABLE

AS

RETURN

SELECT bb.level , 'Myvalue5' as'Table'

From BaseTable bb

Left Join dbo.a on (bb.col1 = a.col1)

UNION

SELECT bb.level , 'Myvalue4' as'Table'

From BaseTable bb

Left Join dbo.a on (bb.col1 = a.col1) ;