内置函数----整理、例题 、xmin

-----------数值函数
---绝对值
select abs(-123) from dual;

--求模
select mod (12,5)  from dual;

--取整
--上限值
select  ceil(123.45) from  dual;

--下限值
select floor (123.45) from dual;

--四舍五入
select round(123.55) from dual;

select round(123.5267,2) from dual;--参数2表示小数点保留几位数

select round(126.5267,-1) from dual;--参数-1表示小数点向左移动1位

--截取,直接舍掉
select  trunc (123.52)  from  dual;
select trunc (126.5267,2) from dual;

select cno, round ( avg(degree) ,2) from score group by cno;


--计算字符串长度
select sname, length (sname)  from student;
select * from  student where length (sname)>2;

--去空格
select trim (‘    付 s十b 亮    ‘)  from dual;
select ltrim (‘    付 s十b 亮    ‘)  from dual;
select rtrim (‘    付s十b亮    ‘)  from dual;

--替换
select  replace(‘  abc   def   ‘,  ‘ ‘  ,  ‘#‘ )from dual;--把空格换成#

select  replace (sname, ‘王‘， ‘李‘) ,sname from  student where sname like ‘王%‘;

--更新姓王的学生名称为姓李的
update  student set sname = replace (sname, ‘王‘， ‘李‘)  where sname like ‘王%‘;

-- 查找字符串
 select instr (‘bac‘,  ‘a‘)  from  dual;  --数据库中索引和字符串位置从1开始
 
 --截取字符串
  select  substr(‘abdjhwkfjf‘, 2) from dual; --从第2位截取后面所有
 select  substr(‘abdjhwkfjf‘, 2,5) from dual;  --从第2个截取到第五位结束
  select  substr(‘abdjhwkfjf‘, -8,5) from dual;  --从右边向左数8位向右截取5位长度，长度不能是负数
    
select sname, substr(sname,1,1)  || ‘同学‘from  student ；

--成绩为空的替换成0
select t.*, nvl (degree ,0) from  score t;

select t.*, nvl (degree ,100,0) from  score t;--  100是不为空的替换成100，为空的默认为0

--替换
select t.* , decode (ssex,  ‘1‘ , ‘男‘ ,‘2‘,‘女‘ ,‘不知道‘)  from student t;

-- 返回当前用户
select user from dual ;

--1.聚合函数   返回单个值
--记录条数
select count (sno)from student where sclass=95031;

--查平均成绩
select sum（degree）/count (1) from score;
select avg(degree) 平均值 from score;

--合计总成绩
select sum（degree） from score;

--最高成绩
select * from score order by degree desc;

select max (degree)最大值, min (degree)最小值,avg(degree) 平均值 from  score;

--字符串数字之间运算

select cast(‘123‘  as number ) +123 from dual;
----------------------时间函数
--查询系统时间
select sysdate from dual;

insert  into course values (‘8-123‘,‘设计素描‘,‘856‘,sysdate);

select sysdate +1  from  dual;
--增加系统月份
select add_months (sysdate,2) from  dual;
--查询系统本月最后一天
select last_day (sysdate) from  dual;

--22、查询和学号为108的同学同年出生的所有学生的Sno、Sname和Sbirthday列。

select t.sbirthday, to_char (t.sbirthday,‘yyyy-mm-dd hh24:mi:ss‘) from student t where sno=‘108‘

select sno,sname,sbirthday from student  where to_char (sbirthday,‘yyyy‘)=
(select  to_char (t.sbirthday,‘yyyy‘) from student t where sno=‘108‘);

select * from student where sbirthday >= to_date(‘1977-1-1‘,‘yyyy-mm-dd‘);