PL/SQL : How to query for & in SQL*Plus?

Today, I tried to execute a simple query like :

     select '&ab' from dual;

as expected I was prompted to insert value for ab...

So, to query special character & you have several options :

• change DEFINE settings to allow &;

set define off;
select '&ab' from dual;


• define a escape character;

set escape '\';
select '\&ab' from dual;


• don't scan for substitution variables;

set scan off;
select '\&ab' from dual;