What does this SQL mean?

Here's the SELECT:

 ( select chargegroupid from txn where accountid = 12159877 group by chargegroupid having max(posted) > dbo.ufn_lastpaperstatementbefore(12159877, cast('2050-01-01' as date)) ) and reversedref is null )

This is actually a subquery of a much larger SELECT, but the thing that makes this significant is that it seems to be limiting the returned number of rows to a date or a range of dates and I'm trying to get my head around it.

The first part: select chargegroupid from txn where accountid = 12159877 group by chargegroupid having max(posted) > 

I'm grabbing the chargegroupid that has the largest, or in this case, the most recent date which is greater than...

Breaking this down now, one piece at a time:

dbo.ufn_lastpaperstatementbefore(12159877, cast('2050-01-01' as date)) )

dbo.ufn.lastpaperstatmentbefore(12159877...

That function looks like this:

USE [PCAR_Data]
GO
/****** Object:  UserDefinedFunction [dbo].[ufn_lastPaperStatementBefore]    Script Date: 10/03/2016 09:37:54 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[ufn_lastPaperStatementBefore]
    (
      @AccountID BIGINT, @dateParam DATE
    )

	RETURNS DATE

    BEGIN
    	DECLARE @firstStatement DATE
		
		SET @firstStatement = (
						select coalesce(max(cast(statement.exportdate as date)),'1950-01-01') from statement with (NOLOCK)
						where accountid = @AccountID
						and cast(statement.exportdate as date) < @dateParam
						and statementtype = 's'
						and voided is null
							)

        RETURN  (@firstStatement)
		
    END

Select allOpen in new window

This function is going to give me the most recent statement based on the export date.

This last part: cast('2050-01-01' as date)) ) and reversedref is null )

I don't understand...