Proc Sql - Condition inclusion of variable from different tables

[Sara1]

How to do -- Case when then select ( column A from table A) else select column b from table b. 

Thanks 

[Tom]

When WHAT?  How are you joining table A and table B so that both available to reference the CASE?

select 
  case when ( ????) then A.A else B.B end as A_or_B
   from A , B
;

Or are you taling about generating code based on some external condition.  So perhaps the value of some macro variable?  If so then use macro logic to conditionally generate the statement you want to create.

%if &XX= OPTION1 %then %do;
select A from A;
%end;
%else %do;
select B from B;
%end;

[supp]

Is this what you are trying to do?

proc sql;
    select case when <put logic here> then a.column_A else b.column_B end as new_column
    from table_A as a
    join table_B as b on <put join here>;
quit;

[Krueger]

SELECT CASE WHEN A.ID = X THEN A.COLUMN
            ELSE B.COLUMN
        END AS NAME
  FROM TableA A
  JOIN TableB B on A.ID = B.ID

[Reeza]

It depends on the rest of the query - COALESECE() may be an option for example. Is table B already in the query somewhere?