取整（向下取整）
select floor(5.534) from dual;
select trunc(5.534) from dual;
上面两种用法都可以对数字5.534向下取整，结果为5.
如果要向上取整 ，得到结果为6，则应该用ceil
select ceil(5.534) from dual;
四舍五入
SELECT round(5.534) FROM dual;
SELECT round(5.534,0) FROM dual;
SELECT round(5.534,1) FROM dual;
SELECT round(5.534,2) FROM dual;
结果分别为 6, 6, 5.5, 5.53
保留N位小数（不四舍五入）按位数截取
select trunc(5.534,0) from dual;
select trunc(5.534,1) from dual;
select trunc(5.534,2) from dual;
结果分别是 5,5.5,5.53，其中保留0位小数就相当于直接取整了。
数字格式化
select to_char(12345.123,'99999999.9999') from dual;
结果为12345.123
select to_char(12345.123,'99999999.9900') from dual;
小数后第三第四为不足补0，结果为12345.1230
select to_char(0.123,'99999999.9900') from dual;
select to_char(0.123,'99999990.9900') from dual;
结果分别为 .123, 0.123
