41.使用函数得到数月之后的日期：to_yminterval（‘01-02’）表示加上1年2月，不能够到天！！
select hire_date,hire_date +to_yminterval('01-02') as hire_date_new from employees where department_id=20
得到多少天之后的日期：直接日期加数字！
select hire_date +3 from employees where department_id=20
42.一般的时间函数：
MONTHS_BETWEEN ('01-SEP-95','11-JAN-94')--两个日期之间的月数，返回一个浮点数
ADD_MONTHS ('11-JAN-94',6)--添加月数
NEXT_DAY ('01-SEP-95','FRIDAY') --下一个星期五的日期
LAST_DAY('01-FEB-95')--当月的最后一天！
ROUND(SYSDATE,'MONTH') --四舍五入月
ROUND(SYSDATE ,'YEAR') --四舍五入年
TRUNC(SYSDATE ,'MONTH') --阶段月
TRUNC(SYSDATE ,'YEAR') --截断年
43.group语句：和高级的应用语句：
SELECT department_id, job_id, SUM(salary), COUNT(employee_id) FROM employees
GROUP BY department_id, job_id ;
使用having进行约束：
1.group by rollup:对n列组合得到n+1种情况
SELECT department_id, job_id, SUM(salary) FROM employees WHERE department_id < 60 GROUP BY ROLLUP(department_id, job_id);
2.group by cube:得到2的n次方种情况
SELECT department_id, job_id, SUM(salary) FROM employees WHERE department_id < 60 GROUP BY CUBE (department_id, job_id) ;
3.使用grouping得到一行中构成列的情况，只是返回1和0：是空的话就返回1，否则返回0(注意不要弄反了！)
SELECT department_id DEPTID, job_id JOB, SUM(salary), GROUPING(department_id) GRP_DEPT, GROUPING(job_id) GRP_JOB
FROM employees WHERE department_id < 50 GROUP BY ROLLUP(department_id, job_id);
4.grouping sets:根据需要得到制定的组合情况
SELECT department_id, job_id, manager_id,avg(salary) FROM employees GROUP BY GROUPING SETS ((department_id,job_id), (job_id,manager_id));
44.from中使用子查询：返回每个部门中大于改部门平均工资的与员工信息
SELECT a.last_name, a.salary, a.department_id, b.salavg FROM employees a,--下面的地方就是子查询了，主要返回的是一组数据！
(SELECT department_id, AVG(salary) salavg FROM employees GROUP BY department_id) b
WHERE a.department_id = b.department_id
AND a.salary > b.salavg;
45.exists语句的使用：
SELECT employee_id, last_name, job_id, department_id
FROM employees outer--下面的 exists里面的select选择出来的是随便的一个字符或者数字都可以
WHERE EXISTS ( SELECT 'X' FROM employees WHERE manager_id = outer.employee_id);