SQL：函数

b站黑马程序员MySQL课程的学习笔记

1.字符串函数

select concat('hello','mysql');#hellomysql
select  lower('Hello');#hello
select lpad('01','5','-');#---01
select trim(' hello mysql  ');#hello mysql
select substring('hello mysql',1,5);#hello
update emp set workno =lpad(workno,5,'0');

2.数值函数

select ceil(1.5);#2
select floor(1.1);#1
select mod(7,4);#3
select rand();
select round(2.345,2)#2.35
select lpad(round(rand()*1000000,0),6,'0');#随机生成6位数字的密码

3.日期函数

select  curdate();
select  curtime();
select now();
select year(now());
select month(now());
select day(now());
select date_add(now(),interval 70 day);
select datediff('2021-12-1',curdate());#-168

select 
name,
datediff(curdate(),entrydate) as'entrydays' 
from emp 
order by entrydays desc;

4.流程函数

select if(true,'ok','error');
select ifnull(null,'default');

select 
name,
(case worksddress when '北京' then '一线城市' when '上海' then '一线城市' else '二线城市'end) as '工作地址' 
from emp;