What is the string concatenation operator in Oracle?

DECLARE
     a      VARCHAR2(30);
     b      VARCHAR2(30);
     c      VARCHAR2(30);
 BEGIN
      a  := ' Abc '; 
      b  := ' def ';
      c  := a || b;
 DBMS_OUTPUT.PUT_LINE(c);  
   END;

output:: Abc def

There's also concat, but it doesn't get used much

select concat('a','b') from dual;

It is ||, for example:

select 'Mr ' || ename from emp;

The only "interesting" feature I can think of is that 'x' || null returns 'x', not null as you might perhaps expect.

I would suggest concat when dealing with 2 strings, and || when those strings are more than 2:

select concat(a,b)
  from dual

or

  select 'a'||'b'||'c'||'d'
        from dual