RPAD(str,len,padstr)
返回字符串str，右填充以字符串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 RPAD('hi',5,'?'); +---------------------------------------------------------+ | RPAD('hi',5,'?') | +---------------------------------------------------------+ | hi??? | +---------------------------------------------------------+ 1 row in set (0.00 sec)
