Total shot in the dark because there is nothing to work and the description is very vague but maybe you want something like this?

CREATE FUNCTION dbo.Example_function_name

(

@Level int,

@MyValue varchar(10)

)

RETURNS TABLE

AS

RETURN

select DISTINCT bb.level , SomeOtherColumn as 'Table'

From BaseTable bb

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

where bb.level = @Level

and SomeOtherColumn = @MyValue

UNION ALL

select DISTINCT bb.level , SomeOtherColumn as'Table'

From BaseTable bb

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

where bb.level = @Level

and SomeOtherColumn = @MyValue