Execute select statement within an IF clause Oracle - sql-server

Question

This is a query from SQL Server, what I want to ask is, how do I want to get the same result in Oracle, what would be the query?
I need result like this: 'this is query on SQL Server'
declare #a int = 6
if #a = 1
begin
select *
from table_A
end
else
begin
select *
from table_B
end
I have tried like this on oracle but failed
declare
type FF_rec is record(
st_Value number
);
var_ff FF_rec;
begin
var_ff.st_Value:=1;
if 1<=0 then
select * from Table_A;
else
select * from Table_B;
end if;
end;

Answer 1

If it has to be at SQL level, then maybe you can use such a code: depending on a value passed to PAR_WHAT substitution variable, you'll get result from EMP or DEPT table.
Because of UNION set operator, select column lists must match in number and datatype,
i.e.
you can't select e.g. two columns from the 1st select and 5 columns from the 2nd;
nor can they not be matched in datatype, e.g. 1st column of 1st select is DATE, while 1st column of 2nd select is NUMBER
So:
SQL> select empno, ename, job
2 from emp
3 where &&par_what = 1
4 union
5 select deptno, dname, loc
6 from dept
7 where &&par_what = 2;
Enter value for par_what: 1
EMPNO ENAME JOB
---------- -------------- -------------
7369 SMITH CLERK
7499 ALLEN SALESMAN
7521 WARD SALESMAN
7566 JONES MANAGER
7654 MARTIN SALESMAN
<snip>
14 rows selected.
SQL> undefine par_what
SQL> /
Enter value for par_what: 2
EMPNO ENAME JOB
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL>
If you're OK with PL/SQL, then one option is to create a function that returns refcursor. Which one? Depends on parameter you pass to it:
SQL> create or replace function f_test (par_what in number)
2 return sys_refcursor
3 is
4 l_rc sys_refcursor;
5 begin
6 if par_what = 1 then
7 open l_rc for select deptno, ename, job, sal, hiredate from emp;
8 elsif par_what = 2 then
9 open l_rc for select deptno, dname, loc from dept;
10 end if;
11
12 return l_rc;
13 end;
14 /
Function created.
Testing:
SQL> select f_test(1) from dual;
F_TEST(1)
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DEPTNO ENAME JOB SAL HIREDATE
---------- ---------- --------- ---------- ----------
20 SMITH CLERK 800 17.12.1980
30 ALLEN SALESMAN 1600 20.02.1981
30 WARD SALESMAN 1250 22.02.1981
20 JONES MANAGER 2975 02.04.1981
30 MARTIN SALESMAN 1250 28.09.1981
30 BLAKE MANAGER 2850 01.05.1981
<snip>
14 rows selected.
SQL> select f_test(2) from dual;
F_TEST(2)
--------------------
CURSOR STATEMENT : 1
CURSOR STATEMENT : 1
DEPTNO DNAME LOC
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL>