oracle 字符串连接

SQL> select 1 || 2 from dual;
 
1||2
----
12
 
SQL> select 1 || 'a' from dual;
 
1||'A'
------
1a

SQL> select 1 || "a" from dual;
 
ORA-00904: "a": invalid identifier

SQL> select (1 || 'a') 'c' from dual;
 
ORA-00923: FROM keyword not found where expected

SQL> select (1 || 'a') c from dual;
 
C
--
1a

SQL> select (1 || 'a') "c" from dual;
 
c
--
1aSQL> select 1 || 2 from dual;
 
1||2
----
12
 
S