This seems like a bug to me, but maybe I just don't understand why this works the way it does... Here is a simple example:

data x; a=1; b=1; output; a=1; b=3; output; a=1; b=1; output; run;

data y; a=1; c=3; output; run;

data z; merge x y; by a; if b eq 3 then c=99; run;

proc print; run;

Here is the output:

OBS A B C 1 1 1 3 2 1 3 99 3 1 1 99

Now why does observation 3 have C=99 ?? Shouldn't it be 3? If this is not a bug, I can't see why anyone would want it to work this way.

Interestingly, if I change the last assignment line in the "data x" step to: a=2; b=1; output; then the variables are handled correctly (to my way of thinking).

I am using Unix SAS 6.12.

Thanks for any help!