LPAD(str,len,padstr)

返回字符串str，左填充用字符串str中垫到len字符长度。如果str为大于len，返回值被缩短至len个字符。

SQL> SELECT LPAD('hi',4,'??');
+---------------------------------------------------------+
| LPAD('hi',4,'??')                                       |
+---------------------------------------------------------+
| ??hi                                                    |
+---------------------------------------------------------+
1 row in set (0.00 sec)