排序：order by
SQL> select last_name, salary from employees order by salary; 默认升序
SQL> select last_name, salary from employees order by salary desc; 降序
SQL> select last_name, salary from employees order by last_name; 字符串排序
SQL> select last_name, hire_date from employees order by hire_date; 日期排序
SQL> select last_name, salary, commission_pct from employees order by salary desc, commission_pct desc;
SQL> select last_name, salary*12*(1+commission_pct) from employees order by 2; 根据位置参数来排序
SQL> select last_name, salary*12*(1+commission_pct) total_salary from employees order by total_salary; 根据别名进行排序
关键字：
select，from，where，order by
函数:
单行函数,多行函数
有累计误差的要用函数
SQL> select last_name,(sysdate - hire_date)/7 from employees; 多少个星期
SQL> select last_name,months_between(sysdate,hire_date) from employees; 多少个月
SQL> select last_name,months_between(sysdate,hire_date)/12 from employees; 多少年
SQL> select add_months(sysdate,120) from dual; 十年后的日期
SQL> select last_name,hire_date from employees where extract(year from hire_date)=2006; 查找 2006 年入职的员工
SQL> select last_name,hire_date from employees where extract(month from hire_date)=9; 查找 9 月份入职的员工
隐含类型转换
明确类型转换
characer —— to_char（格式化输出） ——>
—— to_number ——> number
—— to_date ——>
其他函数：
NVL(nvl1,nvl2)
若 nvl1 不为空，则返回 nvl1; 若 nvl1为空，则返回 nvl2
NVL2(nvl1,nvl2,nvl3)
若nvl1不为空，则返回nvl2；若nvl1为空，则返回nvl3