隐式：即oracle内部自动 转换的，可以把char varchar2 字符类型自动的转换成数值，或者日期的类型。
隐式转换可能会因为自动转换进而导致出错，推荐显式。
显式：自己手动转换。
隐式转换：
select * from scott.emp where hiredate='17-NOV-81';
select '90.81'+50.21 from dual;
select sal+'100' from scott.emp;
显式的类型的转换函数：TO_CHAR, TO_DATE, TO_NUMBER TO_DATE(char[, 'format_model']) TO_CHAR(number, 'format_model') TO_NUMBER(char[, 'format_model']) to_date : select * from scott.emp where hiredate= to_date( '17-NOV-81', 'DD-MON-RR'); // to_date select to_date('May 9,2015','Month DD,YYYY') from dual; select to_date('18-feb-07','dd-mon-rr') from dual; to_char: select ename,TO_CHAR(sal, '$99,999.00') as sal from scott.emp; //to_char转换，oracle默认是varchar2 to_number: select to_number('970.13') from dual; select to_number('-$12,345.67','$99,999.99') from dual; //to_number
