Conditionally Concatenate 2 Tables in a DATA step with two library names

[Philmingo]

In a data step I would like to conditionally concatenate (stack) two SAS tables.  One of the tables may not always be created and would not need to be stacked.  When I have a blank library or table name, I am getting errors.  Here is the sample code.

LIBNAME LIB1 "Filepath";
LIBNAME LIB2 "";
%LET LIB1_TABLE = table_name;
%LET LIB2_TABLE = ;
%LET USE_LIB2 = N;

DATA STACK_TEST;
IF "&USE_LIB1" = "Y" THEN SET LIB1.&LIB1_TABLE. LIB2.&LIB2_TABLE.;
ELSE SET LIB1.&LIB1_TABLE.;
RUN;

With this setup, I would expect the code to revert to the else portion of the "if-then-else" but I get errors in my log.  

%LET LIB1_TABLE = table_name;
%LET LIB2_TABLE = ;
%LET USE_LIB2 = N;

DATA STACK_TEST;
IF "&USE_LIB2" = "Y" THEN SET LIB1.&LIB1_TABLE. LIB2.&LIB2_TABLE.;
NOTE: Line generated by the macro variable "LIB2_TABLE".
LIB2.
_____
22
201

ERROR 22-322: Syntax error, expecting one of the following: a name, a quoted string, (, -, :, ;, CUROBS, END, INDSNAME, KEY,
KEYRESET, KEYS, NOBS, OPEN, POINT, _DATA_, _LAST_, _NULL_.

ERROR 201-322: The option is not recognized and will be ignored.

ELSE SET LIB1.&LIB1_TABLE.;
RUN;

Does someone have a solution on how to go about conditionally concatenating the two tables in a data step when one of the libraries is not populated because the table does not exist?  I am using SAS Enterprise Guide 6.1 (64-bit). I need to perform other tasks within this data step after getting the data from the table(s) which is why I am not using the Append procedure.  

[novinosrin]

something like this can help i think

LIBNAME LIB1 "Filepath";
LIBNAME LIB2 "";
%LET LIB1_TABLE = table_name;
%LET LIB2_TABLE = ;
%LET USE_LIB2 = N;
%macro t;
DATA STACK_TEST;
%IF &USE_LIB1 = Y %THEN %str(SET LIB1.&LIB1_TABLE. LIB2.&LIB2_TABLE.;);
%ELSE %str(SET LIB1.&LIB1_TABLE.;);
RUN;
%mend t;
%t

[mkeintz · Accepted solution]

IFC nested in SYSFUNC would be my prescription:

data stack_test;
   set lib1.&lib1_table  %sysfunc(ifc(&use_lib2=Y,lib2.&lib2_table,));
run;

%sysfunc around IFN or IFC is a great way to do the equivalent of a macro %IF test in open code.

[Philmingo]

Thanks.  Your solution is exactly what I was looking for.