Oracle入门学习三

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学习视频：https://www.bilibili.com/video/BV1tJ411r7EC?p=26

字符串函数：length、upper、lower、initcap、 concat、instr、replace。

-- dual 常量表，没什么意义，就语法规则
-- 获取字符串长度
select length("我是谁？") from dual;
select length("abcd") from dual;
-- 全部变成大写
select upper("abcdDDDFFF") from dual;
-- 全部变成小写
select lower("DDAFAFA") from dual;
-- 首字母大写化，后面的也会变成小写
select initcap("abcdDDD") from dual;
-- 从第一个字符开始截取三个字符
select substr("123456789",1,3) from dual;
-- 从第三个字符开始截取三个字符
select substr("123456789",3,3) from dual;
--字符串连接函数
select concat("ab","dc") from dual;
--替换函数
select replace("我恨你","恨","爱") from dual;
--查找字符串出现的首位置
select instr("java","va") from dual;

数字函数：round、trunc。

--四舍五入的取小数位
select round(3.14159,3) from dual;
select round(3.14159,2) from dual;
--截取小数位，但没有四舍五入
select trunc(3.14159,3) from dual;

转换函数：to_char、to_date、to_number

--数字转字符串
--L指本地货币
--$指美元
select to_char(5006,"L9999.99") from dual;
select to_char(5006,"$9999.00") from dual;
select to_char(5006.989,"9999.00") from dual;
--字符串转日期
select to_date("2019-05-12","yyyy-mm-dd") from dual;
select to_date("2019/05/12","yyyy/mm/dd") from dual;
select to_date("2019/05/12 12:23:23","yyyy/mm/dd hh:mi:ss") from dual;
--字符串转数字 
select to_number("500")+800 from dual;

plsql基本语法熟悉之后，可以配置一下快捷键提高效率：https://jingyan.baidu.com/article/215817f7e1efbb1eda1423ef.html

聚合函数：处理多个数据的函数，常见的有max、min、count、sum、avg。

--列有空也不影响
select max(salary) from staff;
select min(salary) from staff;
select max(bonus) from staff;
select sum(bonus) from staff;
select avg(bonus) from staff;
--count(某列)，当该列值不为空才列入计算
select count(bonus) from staff;
--总行数
select count(*) from staff;

分组函数：group by，根据一列或多列分组，使用聚合函数同级该组的数据。

where用来筛选from字句产生的行，group by用来分组where字句之后的数据，having用来过滤分组之后的数据。

--单列分组 
select name,count(*) from staff group by name;
--多列分组
select department,salary,count(*) from staff 
group by department,salary order by department desc;

--对分组之后的数据，再次进行条件过滤，使用having关键字而非where
select department,salary,count(*) from staff 
group by department,salary 
having count(*)<8 order by department desc ;

--对分组之后的数据，再次进行条件过滤，使用having关键字而非where
select department,salary,count(*) from staff 
where salary>40000
group by department,salary 
having count(*)<8 order by department desc ;

 子查询：查询套查询。执行顺序先子后父。

select *
  from student
 where born_date >
       (select born_date from student where student_name = "bibi")