1.聚合函数:
count(); 统计
select count(*) from stu;
select count(*) from stu where score >= 60;
sum(); 求和
select sum(id) from stu;
avg(); 求平均值
select avg(id) from stu; # 21 / 6 = 3.5
select avg(score) from stu;
max(); 获取最大值
select max(score) from stu;
min(); 获取最小值
select min(score) from stu;
2.数值类
rand(); #生成随机数
select rand() from dual;
select * from stu order by rand();
round()#四舍五入
select round(3.1) from dual;
select round(3.5) from dual;
ceil(); #向上取整
select ceil(3.1) from dual;
floor(); #向下取整
select floor(3.9) from dual;
truncate(); #截取小数位
select truncate(3.1234567, 2) from dual;
3.字符串类
ucase(); #转换为大写
select ucase('hello world') from dual;
lcase(); #转换为小写
select lcase('HELLO WORLD') from dual;
left(); #从左边开始截取到指定的位置
select left('1234567890', 3) from dual;
right();#从右边开始截取到指定的位置
select right('1234567890', 3) from dual;
substring(); #从指定位置开始截取指定个字符
select substring('1234567890', 3, 5) from dual;
