Date:         Thu, 8 Feb 2001 13:46:55 -0500
Reply-To:     Doug Dame <dameda@SHANDS.UFL.EDU>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Doug Dame <dameda@SHANDS.UFL.EDU>
Subject:      Re: Behaviour of many-many match merging in SAS
Content-Type: text/plain; charset=US-ASCII

Howard Schreier <Howard_Schreier@ITA.DOC.GOV> is bothered that

>>  the behavior of a many-to-many MERGE within a BY group
>>  is not the same as the behavior of a many-to-many MERGE
>>  in the absence of a BY statement.

I'm snipping his example entirely (mostly because I would have had to include the whole thing), but the end result is

MERGE w/ no by-var

>>   Obs    left    right
>>      1      A        1
>>      2               2

And MERGE with a (subsequently dropped) by-var

>>   Obs    left    right
>>     1      A        1
>>     2      A        2

And concludes:

>>  It seems to me that the two outputs should be the same,
>>  one way or the other. But they are not. If this is consistent
>>  behavior, it is consistent in some rather subtle way.


I think it all makes sense if you assume a "no by-var" merge is in fact merely short-hand for:

   data COMBINE;
   merge LEFT RIGHT;
   by implicit_by-var [valued as _N_]

In that case, each record would be processed as a FIRSTdot. Hence the variable LEFT is re-initialized as 'blank' at the top of the PDV when Mr. SAS recognizes that there is no incoming record in the LEFT dataset that has a by-value of '2'.


HTH

Douglas Dame
Shands HealthCare
Gainesville FL USA