关于拼接引号的问题

SQL> select ename||q'['aa']' from emp where deptno=10;

ENAME||Q'['AA'
--------------
CLARK'aa'
KING'aa'
MILLER'aa'

SQL> select ename||''''||'aa'||'''' from emp where deptno=10;

ENAME||''''||'
--------------
CLARK'aa'
KING'aa'
MILLER'aa'